Move Box2d to an extension

Long ago, [in a galaxy far away](http://www.badlogicgames.com/forum/viewtopic.php?f=23&t=8268), @badlogic brought up the idea of moving Box2D out of the libgdx core. This pull request removes Box2D to an extension so people who want/need it can add it.

Notes:
* I attempted to create the necessary maven information, but since I am not incredibly familiar with how this works, I may not have done it properly. Someone should check it.
* I made a library descriptor for the extension so it can be included in the Setup-UI, however it will show as a "Third Party" Library due to the Setup-UI not having an "Extensions" section. Is this acceptable, or do we want to try to modify the Setup-UI to have an "Extensions" section (and include box2d, bullet, and freetype as options).
* I moved the ParticleEmitterBox2D into the physics package, to ease the GWT side of things.
* I was only able to test on Windows 64-bit, Android and GWT

Obviously, we'd need to make sure the change was well-known. Thoughts?