Add ability to use an auth token for login

[bigjools]

If using the https backend, you can call ProxmoxAPI.get_tokens() to
receive a tuple of the auth token and csrf prevention token. These
tokens can then be passed to the ProxmoxAPI() constructor to make
a new API object based on the tokens rather than a user name.

api = ProxmoxAPI('host', user='foo', password='password')
auth_token, csrf_token = api.get_tokens()
api2 = ProxmoxAPI(
'host', user='', password='', auth_token=auth_token,
csrf_token=csrf_token)

[coveralls]

Coverage decreased (-1.7%) to 81.081% when pulling 466a9e8 on bigjools:develop into e489195 on swayf:master.