Support requests' AuthBase in addition to basic password authentication
Support requests' AuthBase in addition to basic password authentication

* Why? OWSLib should support more kinds of authentication
  OAuth2/Digest/etc without needing to change further.

* How? Since the `Authentication` object is already plumbed throughout
  OWSLib, just add a new `auth_delegate` property which can be any object
  derived from `requests.AuthBase` and make sure that is honored at the
  points where actual requests' kwargs are built.

geopython/OWSLib,kalxas/OWSLib,tomkralidis/OWSLib