Restore behaviour of cwd in include path

Traditionally Sass has added the cwd as the first include path. This
behaviour was remove in Sass 3.4, and recently in LibSass 3.5.0.beta.2.
People depend on this behaviour and as a result a lot of compilations
are now failing.

This PR restores the expected behaviour and adds a test to boot.

Fixes #1876

Update AppVeyor release config

The AppVeyor config was updated in #1870 but I missed some changes
for the release config.

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