Updated AGALMiniAssembler from latest Starling. Fixed somew untyped
member access in it (casting to String mainly).
Some API improvements for BGScroller class.
Added time accessor to PreciseTimer class.
Fixed Context3D dispose Exception by disabling disposing of screen
manager (and therefore Stage3DProxy) when app is shut down. Can be
neglected since it doesn't have any effect on the host OS.
Added support to screen manager and screens to open screens with a
delay.