Added PausableCrowdsale contract (#832)

* Added PausableCrowdsale contract

* Changed inheritance order to prevent "Linearization of inheritance graph impossible" error

* Updated mock PausableCrowdsaleImpl to new constructor syntax

* Broke function definition to multiple lines

Comply with new max line length

* Rename events to past-tense in PausableCrowdsale test

* Removed should.be.fullfilled from PausableCrowdsale tests

* Change import assertRevert to require in PausableCrowdsale tests

* Remove dependency on chai-as-promised and added BigNumber support in PausableCrowdsale tests

* reindent solidity with 4 spaces

* add missing view modifier in _preValidatePurchase

* convert assertRevert to new shoulFail helper

* add new setup helper

* use expectEvent

* convert to assert to chai should style

* add description to beforeEach blocks

* extract common step to beforeEach

* improve documentation

* revert inheritance error

* move PausableCrowdsale into crowdsale/validation

* make documentation more specific

* put whitespace in line with convention

* improve test suite account names

* undo beforeEach descriptions

* simplify tests

* fix transaction senders to be the anyone account

* make transaction senders more explicit

* remove mocha only

contracts/crowdsale/validation/PausableCrowdsale.sol

+pragma solidity ^0.4.18;
+
+import "../Crowdsale.sol";
+import "../../lifecycle/Pausable.sol";
+
+
+/**
+ * @title PausableCrowdsale
+ * @dev Extension of Crowdsale contract where purchases can be paused and unpaused by the pauser role.
+ */
+contract PausableCrowdsale is Crowdsale, Pausable {
+
+    /**
+     * @dev Validation of an incoming purchase. Use require statements to revert state when conditions are not met. Use super to concatenate validations.
+     * Adds the validation that the crowdsale must not be paused.
+     * @param _beneficiary Address performing the token purchase
+     * @param _weiAmount Value in wei involved in the purchase
+     */
+    function _preValidatePurchase(address _beneficiary, uint256 _weiAmount) internal view whenNotPaused {
+        return super._preValidatePurchase(_beneficiary, _weiAmount);
+    }
+}

contracts/mocks/PausableCrowdsaleImpl.sol

+pragma solidity ^0.4.18;
+
+import "../token/ERC20/ERC20.sol";
+import "../crowdsale/validation/PausableCrowdsale.sol";
+
+contract PausableCrowdsaleImpl is PausableCrowdsale {
+    constructor (uint256 _rate, address _wallet, ERC20 _token) public Crowdsale(_rate, _wallet, _token) {
+    }
+}

test/crowdsale/PausableCrowdsale.test.js

+const shouldFail = require('../helpers/shouldFail');
+
+const PausableCrowdsale = artifacts.require('PausableCrowdsaleImpl');
+const SimpleToken = artifacts.require('SimpleToken');
+
+require('../helpers/setup');
+
+contract('PausableCrowdsale', function ([_, pauser, wallet, anyone]) {
+  const rate = 1;
+  const value = 1;
+
+  beforeEach(async function () {
+    const from = pauser;
+
+    this.token = await SimpleToken.new({ from });
+    this.crowdsale = await PausableCrowdsale.new(rate, wallet, this.token.address, { from });
+    await this.token.transfer(this.crowdsale.address, 2 * value, { from });
+  });
+
+  it('purchases work', async function () {
+    await this.crowdsale.sendTransaction({ from: anyone, value });
+    await this.crowdsale.buyTokens(anyone, { from: anyone, value });
+  });
+
+  context('after pause', function () {
+    beforeEach(async function () {
+      await this.crowdsale.pause({ from: pauser });
+    });
+
+    it('purchases do not work', async function () {
+      await shouldFail.reverting(this.crowdsale.sendTransaction({ from: anyone, value }));
+      await shouldFail.reverting(this.crowdsale.buyTokens(anyone, { from: anyone, value }));
+    });
+
+    context('after unpause', function () {
+      beforeEach(async function () {
+        await this.crowdsale.unpause({ from: pauser });
+      });
+
+      it('purchases work', async function () {
+        await this.crowdsale.sendTransaction({ from: anyone, value });
+        await this.crowdsale.buyTokens(anyone, { from: anyone, value });
+      });
+    });
+  });
+});