script protobuf generation

Since protobuf generation relies on a particular version of go libraries `code.google.com/p/gogoprotobuf/{proto,protoc-gen-gogo,gogoproto}`, we need to vendor them so that we can build the bindings consistently. We do something similar in fleet when generating the API bindings with a bit of [Godep mangling](https://github.com/coreos/fleet/blob/master/contrib/schema_generator_import.go)