google/K2 — pull request #24 by darylseah (2014-07-08 18:08:09+00:00)
Title: Add key version registry.

* Registry for managing the available key version implementations.
* Enables the storage system to easily create a KeyVersion.Builder for a given type.
* Also maintains the protobuf extension registry (contains all the extensions declared by the key version protos).
* Registry and its accompanying RegisteredKeyVersion class are thread-safe.