- Moved the AvroProtocol class into a protocol sub-package.
- Updated the union produced by Option[T] to match conventions in the Avro spec for nullable fields (null comes first in these unions, corresponding to an index of zero).  Updated README doc for same.
- Added static support classes for protocol handshakes (MD5, a handshake match enumeration, and HandshakeRequest/HandshakeResponse record classes).
- Moved AvroProtocol tests into a new separate file, added tests for the new protocol support classes in same.