- 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.
Fixed an oversight in the definitions of the HandshakeRequest and HandshakeResponse classes.  Previously, the metadata was defined as a bytes map, instead of a union of a bytes map with null.