Add valueToPx to public methods

Useful if developer wants to pull what a position a value on the slider will be at. In my case trying set pips at values in slider, without this I can't find a trivial way to get this value.
Add value_to_px public method

See #https://github.com/lokku/jquery-nstslider/pull/15
Update documentation to include value_to_px
Merge branch 'tony-patch-1'