Fixes #22: Make touch-slide speed on mobile devices adjustable

two new options are added (which only apply in the HTML5-mode):
- slidespeed (default 1): A factor by which the sensitivity slide is decreased, i.e. if the slidespeed is set to 2, a touchmove by the same distance will result in half the movement on the cover-flow as in slidespeed 1. When setting this to a value lower than 1, the sensitivity is increased.
- preventslideback (default true): with the current implementation it is not possible to slide backwards without releasing the finger. This may not be desired in some cases.