Code generation tool to support plugin writing

[myitcv]

Depends on #23.

Write a tool that takes a plugin definition (from #23) and code generates the necessary type and MSGPACK wrappers. example/gen_example.go has for now been generated by hand and example/gen_example_gen.go represents the msgp go generate output from the former

Will use msgp to code generate any MSGPACK marshal/unmarshal functions