Verify changed refLow and refHigh before using them.

If the variables scope[refLow] or scope[refHigh] are updated with illegal values
(i.e. if they are outside of the allowed range), the slider behaves weird.
Verifying & sanitizing the variables before using them is therefore a good idea.
Merge branch 'fix/out-of-range'

Conflicts:
	angular-slider.js
	angular-slider.min.js
	src/angular-slider.coffee