Fix slider steps

As reported in #1073 modulus operator `%` can return unexpected results.
This PR addresses that and corrects a bug that  didn't allowed to have
steps with decimals, ie only allowed integers.

Behavior before (`range [0, 5], steps 10` - decimal steps are ignored):
http://jsfiddle.net/2vTqr/2/
Corrected behavior (decimal steps are not ignored):
http://jsfiddle.net/2vTqr/3/