Fix mixed cluster vclock encoding problems

This fix does a few things:
* Removes base64 from the riak_object codec functions. It's used by code
  that wants to send the binary encoded vclock over text explicitly
(JSON and HTTP).
* Normalizes interface to codecs: you encode and actual vclock and get a
  binary, and decoding this binary returns a vclock, however they are
  implemented.
* Added vclock codec round trip unit tests