Changes to signed ints, disalllows step 0

Changes all of the unsigned ints to ints to allow for ranges such as
range(-10, 0) or countdown ranges like range(0, -10, -1).  Additionally
a check for a step of 0 is added.  If a step of 0 is provided to the range
constructor, a RangeError is thrown.
Changes RangeError to RangeException