KroneckerDelta in discrete summations and products

This is a little bit like deltaintegrate is to DiracDelta. Fixes the following issues:

Handle KroneckerDelta in Sum
http://code.google.com/p/sympy/issues/detail?id=3481

Handle KroneckerDelta in Product
http://code.google.com/p/sympy/issues/detail?id=3482