Adjust type of content_type() to include binary(). Closes #53.
Adjust xref checks to ignore unused exports (public API).
Match return values from sending packets.

Resolves:

riakc_pb_socket.erl:1978: Expression produces a value of type 'ok' | {'error',atom()}, but this value is unmatched
riakc_pb_socket.erl:2015: Expression produces a value of type 'ok' | {'error',_}, but this value is unmatched
Merge pull request #174 from basho/bugfix/sdc/dialyzer-xref

Cleanup dialyzer and xref warnings