Added missing methods from Matrix4 GWT emulated class

When attempting to build the html module using the libgdx maven archetype, I encountered errors with Matrix4's emulated class missing methods which are in the gdx Matrix4 class. 

This commit added the missing methods. It also updated a few instances of "angles" to "degrees" (mirroring changes to the gdx Matrix4 version).

First pull request to libgdx, please be gentile :)