Convert \exp{...} to generate e^{...}

Convert \exp{...} to generate e^{...}

Mom wasn't use what exp(...) meant, but she recognized the e^{...}
notation.  I'm not completely comfortable with how this looks in
nested levels during the Gumbel manipulation, but at least it's
consistent ;).  I may tweak the macro if I change my mind ;).

One sneaky bit is the factorials in the \exp{x} nomenclature entry.
Searching around, I was bailed out by the interwebs:

On 2007-08-21, Nicola Talbot <nlct@cmp.uea.ac.uk> wrote:
> ! is a makeindex special character. I haven't used nomencl, but I
> expect it requires makeindex.  Try "! instead of !