Update jQDateRangeSliderHandle.js

Using negative Dates (BC), e.g. new Date(-400,0,1) as bounds crashed the whole slider in Firefox.
In Firefox the following will create an "Invalid Date": var a = new Date(-400,0,1); var b = new Date(a);
So to clone the Date Object it is safer to use its valueOf than the Date-object itself.
Merge pull request #177 from mcauer/patch-1

use ValueOf for creating new dates