Upgrade protobuf messages to 2.6 for python3 compatibility

Updated protobuf messages were generated using protoc 2.6. I had to modify the messages to use relative imports -- but that's pretty minor. Internal changes to the library means that the messages are not compatible with older versions of protobuf. 

One more thing required to finish out #6 -- namely, protobuf needs a python 3 compatible release. I have a patched version.