Project Restructure

- reorganize around a standard gradle directory structure
- create gradle build files
- separate Android specific utilities from the physics core
  to allow use on other java platforms (processing)
- create a continuous integration build script for travis CI
- move to proper semver versioning. The API for rebound is not fully
  finalized we should be at version 0.3
- API is unchanged

TODO:
- maven hosting for build artifacts to allow easy integration