now using RectangleTexture for 'Texture.empty()', as well (closes #354)

Attention: arguments of 'Texture.empty()' have changed!
updated path to my local AIR sdk
minor documentation addition
fixed background loading in demo and scaffold
moved 'backBufferWidth/height' from RenderSupport to Starling instance

This makes it easier to use custom RenderSupport objects.
added method to make screenshot of the stage, saved into BitmapData object
enhanced custom error class descriptions
added some more sanity checks
removed filter margin for full-screen effects (closes #351)
enhanced margin calculation to avoid hard edges
now allowing batching of QuadBatches via the new 'batchable' property.

Right now, used by the TextField class for short texts (less then 16 chars).
replaced automatic TextField batching with "batchable" property users can activate manually.

Otherwise, non-batchable texts (e.g. because the font texture is not in the atlas) would actually be rendered slower than before.
optimization: avoiding assignment by using if instead of ternary operator
small code changes to avoid warnings/errors in FDT
optimized broadcast of ENTER_FRAME event (closes #311)

This solution doesn't change the behaviour in any significant way, and it should be absolutely safe when it comes to garbage collection (even when people forget to call "dispose" on the object).
only listening to context loss event when context loss handling is enabled
workaround for old compiler (refs #311)
avoiding problems when ENTER_FRAME event list is modified while being enumerated (refs #311)
avoiding some more object allocations
added byte arrays to AssetManager; added dispose/clear calls where possible