Create an in memory auth store library

[rgbkrk]

In building out a Rackspace authentication store, it would be quite handy to have a light framework for the operations in auth-store or at least a light framing for providers.

Otherwise I'm writing my own Context and adding stuff on.

[smashwilson]

Unfortunately, Go doesn't make it easy to do this. I made some token effort to share the Context, BindContext() stuff between cloudpipe proper and here, but because the types are different, you can't do it without resorting to interface{} and casting everywhere, which ends up being uglier than the copy-paste.

Such is Go I'm afraid. If I'm wrong I would heartily welcome a PR to the contrary ✨

If you do find anything that can be abstracted out, I'd probably put it in a package like github.com/cloudpipe/cloudpipe/authlib rather than tying it to auth-store specifically. It would also be decent to start a new repo called cloudpipe/auth-base or cloudpipe/auth-lib.