Rearranged into multi-module project

A few advantages:

- I can re-use the scala-sweeteners for JGit elsewhere.
- The BFG uber-artifact will be correctly recognised by maven, so I can
use:

http://repository.sonatype.org/service/local/artifact/maven/redirect?r=central-proxy&g=com.madgag&a=bfg&v=LATEST

Note that unfortunately the useless parent project is getting published
as well, have not been able to stop that happening, despite adapting
this:

http://stackoverflow.com/questions/8786708/how-to-skip-package-publish-with-multiproject-in-sbt