Upgrade Protobuf::RSpec to work with Protobuf 3.0

Up the minimum Protobuf version to 3.0.0.alpha. With the dispatcher now part of the middleware stack and the service now being initialized with an env object, rework `local_rpc` to invoke the service directly.

Also add a few more helpers for testing the full RPC stack: `rpc`, `rpc_env`, `wrapped_request` and update the README accordingly.

Currently pointing at `protobuf/master` until a version of 3.0 is released.

// @localshred @abrandoned