Convert ProtocolBuffers::Message and ProtocolBuffers::Service into modules

Hi guys,

I appreciate it might look like a solid change in terms of gem usage, but I believe it is pretty logical and ruby-way one: the idea of using Protocol Buffers codegen'd classes and stubs breaks really quickly as long as we enforce a base class on an object. A trivial (but pretty real-world one) example would be adding Protocol Buffers serialization to Active Record models.

The change, however, turned out to be really smooth in terms on code change – didn't even have to really fix any of specs (apart from one which created an anonymous Protocol Buffer message instance).

Looking forward to hearing your feedback on this idea :)