Fix threshold property documentation - pixels to points

Changed documentation of threshold property - the property is defined in points (not as pixels). That is:
"threshold of 100.f means that you have to pan view 101.f points", that is 101 pixels on non-retina iPhones and 202 pixels on retina ones.
Merge pull request #16 from bluesm/patch-1

Fix threshold property documentation - pixels to points