Patched iOS error with `drag_start_func_touch`

When no value bar defined, `drag_start_func_touch` would throw an
error on iOS devices. Updated method to test touch location against
the slider, instead of the value bar.
Merge branch 'patch/ios-touch-errors' of github.com:thetron/jquery-nstslider into thetron-patch/ios-touch-errors
Merge branch 'thetron-patch/ios-touch-errors'