Merge readgroups and readsets

- removed `comments` in favor of more generic `tags` like on Reads (where comments can be put easily)
- made the id field non-optional
- added some field comments
Remove flowOrder, keySequence, processingProgram
Attempt to fix build
Merge remote-tracking branch 'upstream/master' into sets--to-groups

Conflicts:
	src/main/resources/avro/ga4gh.avdl
Merge pull request #35 from cassiedoll/sets--to-groups

Merge readgroups and readsets

Merging commit based on community consensus.