TODO(xzyfer): fix this post Node 8

Update module version for node v8.x

V8 5.7 landed recently in node core, causing the module version to get bumped.

Add Node 8 to the support versions lists

Use hardcoded process values rather than inspecting the current
environment.

This is a workaround for nodejs/citgm#389. The install/build flow
needs to be updated so that unsupported environments result in an
informative warning instead of a failure.

Closes #1838
[skip ci]

* Update request version to avoid npm warn

Fix npm rebuild

Running `npm rebuild node-sass` as we instruct people, has been
noop for a while. I'm not entirely sure what has changed.

Debugging as reveal that running `rebuild` now just triggers the
`install` and `postinstall` scripts. The `build` script is no
longer run. This suggests something has changed in newer versions
of npm.

Since the `--force` flag is required to rebuild the binary I've
update the `build` script to take into account if `--force` was
passed to npm.

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