Allow symlinked output directories

Bump npm deps

get-stdin wasn't bumped to ^5 because it introduced a breaking
change which would require use to drop support for node 0.10 or
add a promise library just for this module.

Update contributing docs.

Handle sass.NULL returned from the importer via done()

Whenever done() is called we should check for the sass.NULL
value as well.

While here, allow calling done(null) to anticipate
removal of the sass.NULL one day.

Fixes https://github.com/sass/node-sass/issues/1296

Add test coverage for existing custom importer return semantics

Custom importers are expected to return `sass.NULL` to noop.
However it appears we currently also allow `null`, `false`, and
`undefined`.

This patch adds test coverage over these caveats in case we
unintentionally break BC.

Less V8 bureaucracy for Nan::ThrowTypeError

Use assert.strictEqual in tests for better error output

Use https:// to check out libsass code

When the test fails, we are getting actual
values in the error message instead of

+ true
= false

Update mocha dependency to >= 2.3.4

Disable more flakey watcher tests

Update README URLs based on HTTP redirects

This PR disables the final(?) flakey watcher tests until we have
time dig into the race condition causing failures. Having these
tests randomly failing builds is a terrible experience for
contributors.

That latest version includes an update (mochajs/mocha@039a8a7) to a newer version (2.2.0) of `debug`, which in turn has updated (visionmedia/debug@adc2402) its dependency on `ms` to 0.7.1 in order to include a [security fix](https://nodesecurity.io/advisories/46).

Untested!

Ideally, please also do a point release including this fixed dependency.

Bump LibSass to 3.3.2

Disable randomly failing watcher tests

Until we can commit time to figuring out why they're failing randomly.

Disable randomly failing watcher tests

Until we can commit time to figuring out why they're failing randomly.

This is a work around for sass/libsass#1719 which removed a header
that broke compilation on VS 2013.

Adjust incorrect spacing in README

Minor spacing error causing "file" to read as "f ile". Fixed.

Use the last stable nvm, not the master version

The nvm version from master is unsupported and may be broken. Using a tag
is advised.

CI: Build for node 5.x

Added a test for #1219