Add a new provider: Aad

[kshitij-kasliwal]

[dsyer]

It would be useful to have some background here. What is AAD? Why do we care? Adding support for other providers is perhaps an interesting idea that we could implement as additional projects. We certainly don't want to replace the original facebook and github samples though, so this PR in its current form is unlikely to be applied. If you'd like to re-work it into a new sample app, and explain why you are doing it, that might be a good idea.

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[willmadison]

@dsyer AAD stands for Azure Active Directory. What I believe this PR is trying to accomplish is to make the existing Github and Facebook examples work for Microsoft Azure. We're running into the same issue on a project right now. Namely that the /token request to Microsoft Azure requires some additional fields in the body of the post. Specifically: client_id, resource, & client_secret. All of which seem to be omitted by default. Is there a more idiomatic way to enhance the token request on the fly? Could you point us in the direction of a solid example using Request Enhancers?