Uses native windows API for file IO with Unicode paths

Shortnames may become obsolete or can be disabled.
This implementation should be more future proof!
Reorganizes around try/catch to avoid clang warnings

Calling error in catch will throw, but clang doesn't know!
Adds performance optimization to UTF16 converters
Updates sass2scss to latest tagged version (0.9.1)
Fixes file handle closing on file reading

Forgotten to close the handle in windows implementation.
Fixes to-upper and to-lower to skip utf8 code points
Updates sass2scss to latest version
Merge branch 'master' of https://github.com/mgreter/libsass