ACL file.

Added function to read authorization policy from a file.
Added implementation of Authorize based on that file's policies.
Unit test new authorizer.
Update integration tests to use new authorizer type.
Added docs on authentication and authorization.
Is there a simple abac file that could exist in this directory as a fixture or example for easier comprehension?  I.e. something that looks real that the abac_test.go can call?  Then we have something to point users to.