CI: Use env.SKIP_SASS_BINARY_DOWNLOAD_FOR_CI

Fixes #537.

Change `--output` to be a directory

This makes it easier to watch a lot of files for changes and compile them to a
desired directory rather than writing them in `process.cwd()`.

Cleanup tests

add environment variable to force a build

this can be used to workaround #517

Added test for an issue found in the output of stats.includedFiles

Apply correct cflags to source files

Using -std=c++0x is sufficient to compile libsass

Libsass: Submodule now points to sass/libsass

Add test for watched files rendering

Set `options.src` to render the changed file when watching

Add test for watching files

0.6.4 doesn't include pre-built binaries which makes it incompatible with systems
without Python installed.

Fixes #518

Install: Bind to specific binaries commit

Add ability to watch for files recursively

Add tests for indented syntax and some tweaks

Fix `--watch` option for directories

Switches out `node-watch` for `gaze` since it's used more widely

Fixes #250 and #312.

Submodules: Updated submodules (#510)

* Additions in the binding.gyp.
* Updated CLI test for line endings.
* Updated Appveyor.yml as per @FeodorFitsner's
  suggestion.

Revamp build steps

* Instead of providing the binaries with the package, it'll only download the
needed binary for the users system.
* Will only build if the tests fail.
* Move `build.js` into scripts since it's not part of the actual API.
* Move binaries to `vendor` folder to keep it separate from the rest of the code.

Fixes #504.