Reintroduce typecasting to the Protobuf serializer

The Protobuf serializer used to handle coercing values to Protobuf field types. It was (incorrectly) removed, which broke things that relied on passing values directly to non-persistance methods (i.e. creating requests with values other than the internal attributes) or values directly from web requests (i.e. the Rails params hash).

Reintroduce typecasting using the Active Attr typecasters. Add specs to ensure things work as expected. Fixes #18.

// @abrandoned @localshred