AuthZ authentication

[c0c0n3]

PR #39 implemented pretty much everything we need for AuthZ support except for authentication with AuthZ, i.e. the adapter should authenticate with AuthZ beforehand. As far as I understand this is not strictly needed for the beta release and is only relevant if AuthZ gets deployed outside of the mesh?

Below is @gboege's note about it, taken from the doc detailing AuthZ requirements for the beta release---like I said, #39 implemented all of that stuff, except for the below

QUESTION:
One thing, that just came to my mind is: We might need authorization for AuthZForce.
Please have a look at the function "authenticate" https://github.com/ging/fiware-pep-proxy/blob/master/lib/idm.js.
Straight forward code, plus also a bit caching and configuration.