Add oauth2 support

[lingster]

Add Oauth2 support and examples/documentation.

[saundersmatt]

I'm currently django-alexa with an Oauth2 resource protected by django-oidc-provider.
https://django-oidc-provider.readthedocs.io/en/latest/sections/oauth2.html#protecting-views

Calling the URL requires passing the accessToken gained from session['user']['accessToken'] within an intent. This is the passed in html headers with headers = {"Authorization": "Bearer {}".format(access_token)}

[saundersmatt]

Also stumbled across edx-drf-extensions which meets my particular use case of a remote Oauth2 authentication server pretty accurately, https://github.com/edx/edx-drf-extensions/blob/b46d20a9dd446eae1e6dd38127222ff55cf23ecc/edx_rest_framework_extensions/auth/bearer/authentication.py#L16

I think we need to clarify what we mean by "Oauth2 support" as there's a wide range of things that could mean.