Use Gradle to build LensKit

**This is not ready for review. It is a PR to start CI builds.**

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:

- [ ] Support Maven uploading
- [ ] Upload JavaDocs (and other reports) to dev site
- [ ] Turn on some linting (e.g. checkstyle)
- [ ] File header verification and formatting
- [ ] Create `lenskit-maven` repository to house archetypes and Maven plugin
- [ ] Remove old build system