Add const correctness for sass_values

This should help to avoid issues like #689
Compiler should emit a warning in that case
Merge pull request #692 from mgreter/fix/const_sass_values

Make Sass_Values const in API function calls
Improve color token operations
Fix weird issue with "unquote(0xf00)"
Merge pull request #718 from mgreter/fix/color-token-ops

Improve color token operations
Implement emitting utf8 charset when seen in output
Merge pull request #717 from mgreter/feature/utf8-charset

Implement emitting utf8 charset when seen in output
Fix bug with custom importer and undefined behavior

Worked on windows but fails on linux, since strdup(0)
is undefined behavior. Avoid calling it in that case!
Bubble @return in each lists
Merge pull request #737 from xzyfer/fix/return-each-lists

Bubble @return in each lists
Parse selector identifiers prefixed with an ampersand correctly
Merge pull request #739 from xzyfer/fix/bem-style-identifiers

Parse selector identifiers prefixed with an ampersand correctly
wip