enable http authentication in pydas api

[mgrauer]

Currently http authentication is available in some methods, but not really at the top level api of pydas. Think about how to provide this in the credential provider methods.

This can be accomplished by a call of

pydas.session.communicator.set_auth(("username",'password))

We need to take in auth as an optional keyword for the credential provider methods, or possibly all the public api methods. This could be accomplished in the login method:

https://github.com/midasplatform/pydas/blob/master/pydas/api.py#L39

Also, the lower level download_item (and possibly other) calls will need to have parameters updated to include auth.

https://github.com/midasplatform/pydas/blob/master/pydas/drivers.py#L406