Fix handling of touchmove events on Android

Dragging the canvas did not work on Android devices as the === check for
lastTouch and the current event was always evaluating to false.
Presumably Safari on iOS re-uses the same Touch object for touchmove
events with the same finger, whereas Chrome/Firefox on Android creates
new Touch objects for each event (so the === evaluates false). The code
now compares Touch.identifier to ensure the new touch event is from the
same finger as the initiating touchstart.
Merge pull request #19 from h4l/android-compat

Fix handling of touchmove events on Android
refs #4 - have noticed navigator is often blurry when rendered.  changing blendTime can alleviate or worsen this issue if it is more or less than the animationTime (respectively).  Im setting this to the value equal to animationTime as a hot patch while we figure out why these two variables are coupled.
Merge branch 'master' of https://github.com/openseadragon/openseadragon into requestanimationframe