initial tagging and marshaller for extendability

Also includes a handful of fixes to the (renamed) Coder.

Indexing on an int type using riak protobufs seems to be broken since the internals are of type []byte.  The server throws back a pre-commit error.  I'll post to the email list or ask in irc about this at some point.

Next commit will be the Unmarshaller logic.
coder unmarshaller. two new core methods
Merge pull request #22 from boj/marshaller

initial tagging and marshaller for extendability
slight cleanup