Initial support of GPX format

Adds initial support for the GPX format. GPX file decompressed, score.gpif extracted from resulting file system and parsing started. Support for different instruments/bars, notes from all non-percussive parts (support for *some* percussive parts), dotted rhythms, initial support for tuplets, and more. Warnings are generated when information is simply skipped over, these should reduce with future pull requests. When support is a little better developed and less likely to change I'll commit some of the tests I have for this. Any comments/questions on the below fire away!