Patched iOS error with `drag_start_func_touch`

We're working on a project at the moment which makes heavy use of nstSlider on iOS devices. We noticed that when using the slider in single-handle mode, with no 'value bar', that it would not respond to touches, or dragging.

I have modified `drag_start_func_touch` to test the touch location against the actual slider itself, rather the value bar, and it appears to work as expected now on iOS.

I also wanted to include a failing test here - but JavaScript testing is not my strong suit and I couldn't figure out how to do it without stubbing some methods (which, again, I was a bit clueless about here).