Update to handle small steps - ie: 0.0000001

I was having issues with very, very small step increments.  Updated so that the code now will handle small steps by multiplying by that step amount to convert to a handleable number.

Example: 
	var testObj = ko.observable('')
                    .extend({ step: 0.00000003 });

	testObj(0.00001115);
        testObj.isValid() != true;