Enable ColorMatrixFilter function chaining

This allows, e.g.:
new ColorMatrixFilter()
    .adjustHue(h)
    .adjustSaturation(s)
    .adjustBrightness(b);
add return value to ColorMatrixFilter.concatValues
updated release date
set version to 1.5 in maven pom.xml
updated swc file for 1.5 release
raised version to 1.5.1-SNAPSHOT
exchanged references to HTTPStatusEvent.HTTP_RESPONSE_STATUS with custom constant (closes #545)

This constant is not always available in pure Flash.
raised version to 1.5.1
updated SWC for 1.5.1
updated changelog for 1.5.1
updated maven pom.xml for v1.5.1
starting development on v1.5.2
added 'color' property to the Button class
optimized TextureAtlas.getNames by caching sorted list of names (closes #548)
updated README with more information about cross-platform deployment
added method to get all contained char IDs from bitmap font (closes #554)
added 'hasChars' utility method (closes #347)
now conserving 'onReady' texture option when loading ATF texture (closes #555)
avoiding 'slice' when possible (refs #493)
added 'over' and 'disabled' states to Button (refs #549)