Implement 'log' in scheme

[Katie Burke]

(log num) should returns the natural logarithm of num.
(log num1 num2) should return the base-num2 logarithm of num1.
( http://www.scheme.com/tspl4/objects.html#./objects:s129 )

This should return 0.0

(log 1.0)

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

This should return 2.0

(log 100.0 10.0)

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