First pass at adding MS Azure OAuth2. This works (an access token is issued) but needs way mor work.

Cashier configuration:
oauth_client_id: This is the Application ID assigned when you register the app in Azure AD
oauth_client_secret: This is created when you create a new key in the application
provider_opts: Set "tenant" to an AD domain name (e.g. blabbedy.onmicrosoft.com)

TODO:
Users whitelist is untested currently.
Valid() is unimplemented.
Revoke() is unimplemented. Not checked if an API exists on the MS side.
Email() is unimplemented. Should be pretty simple using the graph API.
Needs tests.

Oh and last but by no means least MS issue INSANELY LONG (> 1800 chars!) access tokens which are too long for my shell. This represents a significant issue :/