Exports quote and unquote functions for C bindings

Needed to exchange strings with registered functions.
Fixes an edge case when buffer is empty

https://github.com/sass/libsass/commit/e5a8ea77c340028cf23511ea52cb62505924241c
Fixes issue #541 with negative number output
Merge pull request #542 from mgreter/negative-number-output

Fixes issue #541 with negative number output
Merge pull request #544 from mgreter/fix-trailing-semicolon-removal

Fixes an edge case when buffer is empty
Merge pull request #543 from mgreter/expose-quote-function

Exports quote and unquote functions for C bindings