Adding sequence records, and cleaning up reference records.

@jeromekelleher and I plotted this one out this PM; we think this is a more consistent and general approach to serving references and sequences. Specifically, we:

* Added `GASequence` and `GASequenceFragment` models. These models define a _generic_ contig sequence, which could be used to describe a reference assembly, a _de novo_ assembly, variant calls spliced back into a reference assembly, etc...
* Cleaned up a few fields in `GAReference` and `GAReferenceSet` to make the relations of the models slightly cleaner/clearer.
* Removed `/references/{id}/bases` GET request. Instead, we have added `/sequences/{id}` and /sequencefragments/{id}` GET requests, which are more general.