A rewrite using FFI.

Basic interface is working, advanced features (MCN, ISRC, feature detection) are missing. This also adds some backward incompatible API changes and renames the library to discid.
Added put method, updated unit tests.
Allow setting the features for read
DiscId.new is private now. Instead DiscId.read or DiscId.put should be used directly.
Renamed Api module to Lib.
API refactoring
Support has_feature?
Removed .gitattributes
Started documenting the library with Yards