recognize b**(1/log(b)) and b**(1/(log(-b)+/-I*pi)) as E

The random test added will affirm this identity every time tests
are run. I already ran it several hundred times without error. The
code in power.py tries to fail quickly if it is likely that the
base and exponent don't match the target expression.
Merge pull request #2057 from smichr/pow-E

3788: simplification of powers