v2.2

v2.2 is backward-compatible with v2.1 and adds the following features:

• support for encoding transparent gifs (courtesy of @flaviozavan)
• a set of functions to encode directly to a file, for use on memory-constrained platforms
• support for passing in pixels in BGRA byte order
• improved memory footprint and allocator usage
• improved documentation

Additionally, all the code and data I use for testing is now included in the repo, which should make it possible, and hopefully easy, for contributors to use.