Merge pull request facebookarchive#37 from facebook/bugfix

Convert ReentrantCallback to CopyOnWriteArraySet

bump version to 3.3

origami value mapper

Adding a helper for constructing spring config values that match up with
the Origami design prototyping tool.

http://facebook.github.io/origami/

Programatically generate configurator view

This removes the dependency on resources making it possible to put
the entire project into a jar file for easier integration.

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

Build the sample project with gradle as well