Make the watcher more responsive to child changes

Watching for changes on files with lots of `@import`s had a
significant regression in responsiveness in #1745.

The regression was caused by calling `gaze.add` unnecessarily. We
only need to call `gaze.add` on files that aren't currently being
watched.

At the time I confirmed that calling `gaze.add` in files that were
being watched wouldn't result in a leak or multiple events being
fired. I however assumed calling `gaze.add` for already watched
files would be very cheap.

Fixes #1869

Use Visual Studio 2015 for Node 6+

Node change to Visual Studio 2015 when Node 6 went to LTS.

See nodejs/node#7989
Fixes #1854

Bump LibSass to 3.5.0.beta.2

Remove the git fallback

Remove git sub module documentation

Remove the LibSass git submodule

The git submodule is removed as of #1850.

This was added by me in #874 to support node-sass being installed
as a gitish via GitHub rather than the npm registry.

We publish the LibSass src to npm already so it should continue
to work as is.

I can't imagine accepting a PR with a LibSass bump.

It'd be painful to audit. LibSass should be handled by core.

Commit LibSass source directly from the 3.4.3 tag

* Bump LibSass to 3.4.3

* Update sourcemap test fixtures to match new LibSass behaviour

* Bump sass-spec@v3.4.3

Sass spec releases match 1-1 with LibSass releases and need to be
versioned in sync.

Add a platform variant for installing a musl binary

Fix premature exit before the output stream is fully flushed

Drop dependency lodash.isarray

Bump LibSass to 3.4.0

https://github.com/sass/libsass/releases/tag/3.4.0

Update sourceComments documentation in README.md

Use console for logging instead of npmlog