add .coverage to .gitignore
merge openid and oauthsupport

* if DJANGO_ADMIN_SSO_OAUTH_CLIENT_ID i not set it will use openid
* if DJANGO_ADMIN_SSO_OAUTH_CLIENT_ID is set it will use oauth
* when using openid a deprecation warning will be raised
* models OpenIDUser, Association and Nonce will only be imported if using
  openid
add python-openid to test requirements