Added FadeStateTransition

You can fade in or out with this transition.
Exposed a few rendering methods for GuiService

The new methods allow the client to render a portion of the GUI -- like
the "root" or "hud" layers. This allows game states to render the GUI
when they need to rather than having it always rendered after.
Made game states render their own GUI
Added in/out transition to GameController

This allows custom in and out transitions to be used between states.