[feature] Login/logout without GET/POST

[fcarreiro]

Use case: I have an API where I need to send a basic-auth header on every call, but I don't necessarily need to do a separate GET/POST to login/logout. In this case, it would be useful to have a function that sets the headers and one that clears them. Alternatively, one could check if url is null and skip the $http call.

If you find that this feature makes sense, I can create a PR.