Add support of groups

Hello,  I would appreciate it if you accept this feature.

I am little worried about it because REAME.md says "Probably Never to be Supported" about groups.  Does it mean just you don't want to spend your time to implement the support, or you never want to support it?

I hope you meant the first one.  Thanks!

---
This commit does:
* Extends the Ruby-internal DSL for protocol buffers so that it supports
  groups.
* Lets the complier to generate the DSL for group if the field is a
  group.
* Allows the runtime to serialize and deserialize group fields