add multi-touch support

I've tested it extensively on an iPad mini and behavior is quite good.
Will need to be tested on Android/Windows mobile as well as devices
with a smaller form factor.
remove debug messages
increase zoom speed
implement double-tap
fix TouchHandler creation
android touch WIP
get touch working on Android

The patch is based on the work by @kozmad. It unifies the Android and
iOS implementation to avoid code duplication. I've only tested this
on an iPad, but it should also work on Android.
add docs to touch.js
add multi-touch support to readme
add two semicolons
Merge branch 'multi-touch'