Enable ColorMatrixFilter function chaining

This allows, e.g.:
new ColorMatrixFilter()
    .adjustHue(h)
    .adjustSaturation(s)
    .adjustBrightness(b);
add return value to ColorMatrixFilter.concatValues
fixed handling of invalid image data in AssetLoader (closes #563)
added 'texture' property to BitmapFont class
added some manual dispose-calls for internally used BitmapData (closes #565)
added 'cleanMasterString' utility method and using it whenever parsing XMLs (closes #566)
replaced XML 'attribute' method with literal '@attribute' access for better performance (refs #566)
added ByteArray-fallback if AssetManager can't parse XML or JSON (closes #568)
fixed how fragment filter reuses pass textures (closes #569)

Non-integer content scale factors caused useless texture recreation
fixed that 'Texture.empty' sometimes did not return the requested size (refs #569)
Merge pull request #434 from tconkling/chainableColorMatrixFilter

Enable ColorMatrixFilter function chaining
small indentation fix
enhanced API docs of Button class a little
added 'readjustSize' method, analog to the same method in the Image class (closes #575)
added more API docs to the 'fromTexture' method
moved try/catch around actual instantiation (closes #579)
now dispatching an IO_ERROR when an asset can't be loaded from an URL
removed erroneous call (refs #579)
small simplification
now loading assets in parallel (closes #502)