I reworked the patch a bit as per Mark's comments. It takes care of
the sign part and handles integer power values properly. It also fixes
another bug with the current expt implementation where base is an
integer and the power value is not (eg: (expt 2 2.0) gives an error in
the current git master. The patch fixes this too.)

Also adds some test cases. Please review.

Thanks
Ramakrishnan