Merge pull request #23 from darylseah/master

Updates to Protos and addition of KeyVersionInfo annotation.
Update HACKING.md

Add protocol buffer version 2.5 requirement.
Add key version registry.

- Structure is similar to storage driver handling.
- Completely thread-safe.
- Added appropriate exceptions for the registry.
- Includes some alterations to KeyVerions.
Merge branch 'master' of https://github.com/google/K2