Eval Abs(exp(x*I)) as 1 if x is real

- also rewrite re and im in terms of each other, useful for some simplifications
On Sat, Nov 01, 2014 at 04:18:03PM -0700, zanzibar7 wrote:
>    Sorry. I don't understand the comment. Currently, exp(3*I).is_Pow and
>    exp(3).is_Pow are false.

Hmm, indeed, my bad.  #4898