Key lookup is brittle

Currently when bot.py looks for it's own key it matches very strictly against the configured name and email:

```
expected_uid = '{0} <{1}>'.format(config.PGP_NAME, config.PGP_EMAIL)
fingerprint = gpg.has_secret_key_with_uid(expected_uid)
```

If I happen to change cryptobot's pgp name in config.py it will fail to find its own key. Same if I generate a gpg key with something in the comment field, e.g., "Crypto Bot (TEST) cryptobot@eff.org".

Suggest modifying GnuPG::has_secret_key_with_uid to list all keys for config.PGP_EMAIL and take the first one. For bonus points, add a key id config, just in case you have generated multiple keys for cryptobot.