Jsxtransformer cleanup

I got carried away. I couldn't help myself.

Note: this also removes some APIs that were publicly exposed. I'll spot check react-rails and react-python to make sure they weren't using them. There was no need to make them public anyway.
(I still want to get rid of the docblock for non-FB stuff and just use the .jsx extension and transform always if `type="text/jsx"`…)