Set up gradle

- [x] Switched the project over to use gradle instead of and for building
- [x] Set the project up to run on Travis-CI
- [x] Updated README.md with more info on how to set the project up
- [x] Changed project source roots to match maven / gradle conventions
  - `/src/*   -> /src/main/java/*`
  - `/test/* -> /src/test/java/*`
- [x] Ignored / removed a bunch of files that shouldn't have been in source control