Define GET /reference/{id}/bases in favor of GAReference.sequence.
Improve consistency for reference methods

- `md5checksums` is consistent with `GAReference::md5checksum`.
- Support repeated md5/accession filtering in SearchReferenceSets, to be consistent with SearchReferences.
Merge pull request #129 from calbach/patch-5

Improve consistency for reference methods
Merge pull request #120 from calbach/ref-bases-rds

Define GET /reference/{id}/bases in favor of GAReference.sequence.