Don't rewrite (x^a)^b, unless b is Integer

The formula (x^a)^b = x^(a*b) only holds if b is an integer.
Add a comment about the power simplification
Only simplify (x*y)^b = x^b*y^b if 'b' is an int
Add tests for power simplification

These fail in master, but pass in this branch.
Merge pull request #236 from certik/pow

Don't rewrite (x^a)^b, unless b is Integer