Changes to use freetype-rs instead of rust-freetype in servo

Copied functions and types from rust-freetype into this module.
Moved the TT_OS2 struct to ffi.rs

Moved TT_OS2 struct to foreigh function interface file.
Added a rust equivalent struct - TrueTypeOS2Table with some getters
Changed the struct name back to TT_OS2 in ffi.rs
Merge pull request #70 from ankit3005/master

Changes to include this library in servo