Update a few odds and ends to the latest spec

Per recent changes to the wiki page. A few new error codes, and one new
timestamp field in the OffsetCommitRequest message.

There is one new request/response pair as well (Consumer Metadata) but it hasn't
been given an API Key value so I'm holding off on it until I can get
clarification.
Merge pull request #85 from eapache/proto-updates

Update a few odds and ends to the latest spec