SequenceDictionary adds containsRefName, updates recordName key types.

Added containsRefName to SequenceDictionary, allowing us to check whether a SequenceDictionary object contains a particular sequence.
SequenceDictionary recordNames keys should be Strings (they had been Avro Utf8 objects, which was messing up lookups into the map itself from Strings).
Merge pull request #47 from genomebridge/tdanford/seq-dict-contains-ref

Adding containsRefName to SequenceDictionary