Add libgdx-screenmanager

[crykn]

A screen manager for libGDX supporting transition effects. Handles the changing of screens and allows to apply effects during the transition from one screen to another (screenManager.pushScreen("settings_screen", "blending_transition")). Also supports transition shaders, so all the cool shaders on gl-transitions can be used.

Documentation is provided in the wiki and the repo contains a simple example to get started.

Some of the effects that are possible:

[payne911]

I'm in favor of this project being listed. :)

[crykn]

Thanks, @rafaskb!