Added few functionalities / fixed some bugs

The following functionalities have been added:
- it is now possible to tell slider to disallow swiping by passing disableSwiping in options. This defaults to false
- fast finger movements will now trigger swiping, even if they are very short (sensitivity option controls how sensitive it is. 0 disables this functionality entirely)
- it is possible to initiate slider to display any slide, not just the first one
- added a few unit tests

Bug fixes:
- fixed issue where every other swipe movement would be ignored on some devices
- added touchcancel event handler so disrupted movement still gets handled
- all listeners are now correctly removed