Use issuer instead of hostname in getUrl

An issuer is STRONGLY RECOMMENDED: https://code.google.com/p/google-authenticator/wiki/KeyUriFormat
The label can be an email but doesn't have to be. The hostname was a bit unexpected, and it wasn't possible to use an identifier.

This could be considered as a breaking change, but I changed the hostname to issuer. So instead of having the label username@hostname.com it will now by username, with hostname.com as issuer. That should only effect the way the app shows the token (I think)

If you were to allow a more breaking change, the issuer could be made optional as the last argument, although it is stated as strongly recommended.

(An alternative would be #5 but that doesn't allow for labels other then emails)