Use Gradle to build LensKit

Use Gradle instead of Maven to build LensKit.

This has a few advantages:

- Far easier to do nonstandard build actions (e.g. generate class list)
- Significantly faster build + test times
- Fewer random pieces of glue (Ant, shell scripts, Python scripts, etc.)

Some outstanding tasks:

- [x] Support Maven uploading
- [x] Upload JavaDocs (and other reports) to dev site
- [x] File header verification and formatting
- [x] Create `lenskit-maven` repository to house archetypes and Maven plugin
- [x] Remove old build system
- [x] Use jGit for log generation