Before unpickling anything, ensure that it has a valid digital
signature using a randomly-generated shared key. In order for an attacker
to send or tamper with data on the same socket, they must know this key
to compute a valid signature.

- Bind to localhost instead of 127.0.0.1
- Update key generation method
- Import cPickle if available

- remove redundant call to bytes
- use hmac.compare_digest

- Document _compat_compare_digest