Support protocol buffer code generation

Protocol buffers are a language-independent data structure definition and serialization library.
http://code.google.com/p/protobuf/

They are used extensively at Google and are useful for open source projects as well, especially in multi-language distributed systems.

There's a protoc compiler which reads a .proto file which contains the definitions, and generates code in Java, C++, or Python.

It would be great to have support in Buildr. I think it would be structurally similar to how the Antlr support works.