1318: Interval arithmetic support - First bits

This is a begin to implement Interval arithmetic support, which I require for another issue. Most of the added tests currently fail.

Could someone give me some hints, e.g. implementing this for sympy.core.add please? I would implement it and add tests according to wolfram.

The rules for it are obvious and are documented here: http://reference.wolfram.com/mathematica/tutorial/IntervalArithmetic.html .

Thanks
     Torsten