Make SafeMath a library

Update README with new slack url

Added totalPayments uint on PullPayments contract

Add MintFinished event to MintableToken

Remove excessive condition from SafeMath.safeAdd()

There is no situation when `c>=a` will be `true` while `c>=b` will be `false`.

fix for short address attack

Create and test MintableToken contract

as suggested by izqui in case a function calls transfer under the hood

Change killable to destructible and kill to destroy

as suggested by /u/izqui9 here https://www.reddit.com/r/ethereum/comments/63s917/worrysome_bug_exploit_with_erc20_token/dfwmhc3/
Attack description: 
https://blog.golemproject.net/how-to-find-10m-by-just-reading-blockchain-6ae9d39fcd95

Follow security audit recommendations

Shareable is not used in this contract

Fix vesting calculation logic

Rename `TransferableToken` to `LimitedTransferToken`

fixes #179

Change sha3 and suicide methods to keccak256 and selfdestruct respectively

see ethereum/solidity#1476

Prevent contracts from owning Ether, Tokens and other contracts.