Timing-attack-resistant string comparison

The current implementation leaks information about the correct value of
the OTP code in the string comparison, since python's string comparison
is short-circuiting. This fixes the vulnerability. See
http://crypto.stanford.edu/~dabo/papers/ssl-timing.pdf for why this is
important even in network applications.
Merge pull request #6 from jboning/timing

Timing-attack-resistant string comparison