fixed issue #67: When changing value of slider, slide callback is not firing

Added extra function call of 'call' within the 'val' function, to
trigger the 'slide' event, once slider's value is changed
programmatically.

Note: I wasn't been able to find a better reference to 'noUi-target', within that function scope. So if you feel like there's a better way, feel free to comment.

P.S. You should totally switch to spaces instead of tabs ;)