Fix KeyframeAnimationEffect.toString()

After 6a4d32eedc089e0b69b3b6f9201523bdf47d5313, KeyframeAnimationEffect no
longer has a property attribute.
Remove unused AnimationEffect.getValue()
AnimationEffect.sample() should not be public
Merge pull request #331 from steveblock/keyframeBugs

Fix a few minor bugs in effects