Adding JUnit support.

Changes:
- Adds a waf task all, which runs everything except for configure and deps
- Adds a waf task deps, which downloads dependencies into deps directory
- Adds a waf task test, which runs the KodKod JUnit TestSuite
- Adds sample JUnit test suites and test
- Updates travis config to download dependencies and run tests
- Adds a gitignore

Reviewers: @AtulanZaman @mhyee @joseph2625