Jaxb works with arbitrary package names now, and pants's internal BUILD files have been updated to include jaxb.

Remaining issues:
- No test cases exist (removed the only existing test file, because it testing methods that no longer exist)
- When running jaxb_gen, pants still produces a warning message: "Left with unexpected unconsumed gen targets" despite the fact that it generates the files just fine.
- Jaxb's entry in pants.ini is still bogus, in that the information isn't used even slightly.
- Custom packages seem to work well, however the default behavior of dynamically guessing the package name based on the .xsd path is still kind of a hack. Maybe this is okay though.
- Pants doesn't cache the generated stuff very well, and requires a "./pants goal clean-all" if the .xsd files are changed. Not sure how to fix this, because I don't know enough about how the caching system works.
- I think what I did with SourceRoot may require .xsd files to be listed under src/resources, but I'm not positive about that. Regardless, that's how hoptoad has it, and what was recommended on the Google Doc, so perhaps that's okay. I don't fully understand how the SourceRoot system works, but it was complaining at me before I added that because JaxbLibrary was 'not allowed under src/resources'.