Merge pull request #26 from racarone/master

SPViewController OpenGL context fix
raised version to 2.1
updated change log
fixed that stats display could only be enabled after context was created
fixed filter caching and implemented 'isCached' property
now disabling vertex attrib arrays after filter has been rendered (closes #28)

I'm not entirely sure why, but here it's necessary. Otherwise, after using the DisplacementMapFilter once, Sparrow's performance degrades by almost 20 percent!
added 'touchGroup' property to display object containers, and using it in SPButton
enhanced quality of displacement map filter on iPad 1 (showed artefacts before)
Merge pull request #22 from Gamua/master

Gamua additions