Updated master/master.cfg to sign releases.

Setup now requires GPG to be installed and in the PATH env variable, as well as the following files:
```
rust-bot-sign-secretkey.asc
rust-bot-sign-publickey.asc
rust-bot-sign-passphrase
```

Well, not necessarily the public key if you want to import subkeys yourself.

I put the passphrase in `rust-bot-sign-passphrase` because it seems as if Mozilla already does something similar for other software (see [passwords.py.template](https://github.com/mozilla/build-buildbot-configs/blob/master/mozilla/passwords.py.template). Except that this isn't Python.

**NOTE**: I couldn't get Rust's master.cfg to work locally, so I just made sure that signing worked for a test project of mine.  
**OTHER NOTE**: You'll also probably want to generate a subkey to use.