Add support for protobuf-2.3.0 plugin structure

```
Is there any plans to support the 2.3.0 plugin structure in the near future?  
Also I would suggest to keep the version scheme to match major protobuf 
release, i.e so that 2.3.X signifies compability with protobuf 2.3.x


```

Original issue reported on code.google.com by `kal...@gmail.com` on 13 Jan 2010 at 8:33
