Small comment change about typedef workaround.
Added GitHub Flavored Markdown (GFM) todo list.
Cleanup pass.

- Cleaned up UTFX_API macros.
- UTFX_API now switches on gcc and msvc compilers.
- UTFX_UNREFERENCED uses static_cast instead of c-style cast. This will probably explode under msvc, but we can't compile with it yet anyway.