Implement 'exp' in scheme

[Katie Burke]

exp should return e to the num power.
( http://www.scheme.com/tspl4/objects.html#./objects:s129 )

This should return 2.7182818284590455

(exp 1.0)

Traceback (most recent call last):
File "stdin", line 1, col 2
RunTimeError: unbound variable 'exp'