Use 'keyring' module to get password from OSX keychain or other key store

Rather than having to store passwords in the config file in plain text or get them from the console each time, this change allows users to save passwords to the OSX Keychain or any other backend supported by the [keyring](https://pypi.python.org/pypi/keyring) module.

If there's no password in the config file, it will check for the presence of the keyring module, if it's available, it will see if the password is already saved to the keyring.  If not, it'll get it from the console and save it to the keyring so that it's not required the next time.  If the module's not available, it'll get the pass from the console as before.