Add sass-spec tests

[ndhoule]

Adds the sass-spec test suite. Continued from #222.

I mostly ported over those changes, with the exception of a few fixes:

• Now uses the same CSS formatter used in sass-spec. (Not sure if that's related to the Windows line handling issue noted in [WIP] Sass spec testing #222.)
• Test suite fails if sass-spec isn't cloned into place, or if any sass-spec tests are failing. Seemed like if sass-spec should replace any non-middleware functionality, then test passage should be dependent on sass-spec tests passing.

Interestingly, the test suite was failing at first, but I rebuilt the binary per the readme/#428 and all tests (including the one noted in #427) passed.

Related issues: #161, #424, #427, #428

[am11]

@andrew, LGTM! 👍
AppVeyor test is failing due to some kind of timeout. It is not triggering node scripts/test intermittently for some crazy reason (perhaps their thread / dyno gets sleepy..) /cc @deepak1556

[andrew]

Yeah this is pretty awesome, the binary situation is pretty crappy right now, hopefully we can get it to a better place soon.