Fix simple maps

In doing some work for lumberjack, I noticed that there were some valid Pig schemas that elephant-bird was generating (for example: map[map[int]]) which were not able to be converted. I fixed conversion support (Pig Schema => Message type) though I'm not sure if I will need to fix it anywhere else.

This includes the pull request: make_pigschemaconverter_static. But that is a tiny request so you'll see the changes that dominate.