Added steering behaviors.

See the [wiki page](https://github.com/libgdx/gdx-ai/wiki/Steering-Behaviors) 

**Features:**
- Cross-engine: tests provide basic implementation for scene2d, box2d and bullet
- Support for 2D and 2.5D
- Independent facing
- Over 20 behaviors
- GUI-based tests to get familiar with steering behaviors and their parameters.