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