Adding HeaderPolicy to explain who can do what

This is the first pass. With plenty of magic strings.
Tightening up HeaderPolicy behavior

This more closely reflects the data structure of Models::Permission
Tightening up query
Adding policy behavior

This is a work in progress and something that is not quite right with
what I'm working on.
Adding :policy_authorization_method_name

Runners should be able to enforce this
Adding explicit default for policy method name

I want it to fail loud. A runner always has a policy method, and I
want to clearly convey both failure and when it has been set.
Tidying up authorization enforcement
Creating a BasePolicy

As I need standard methods, I'm interested in leveraging those.
Workign on additional policy enforcement
Adding additional documentation

And renaming methods to be more concise and meaningful?
Collapsing multi-line into single
Making sure policy enforcement happens
Merge pull request #16 from ndlib/adding-authorization-required

Adding authorization required
Verifying CreateHeaderForm#policy_enforcer
Adding BaseRunners#enforce_authorization_policy!
Adding policy authorization
Removing explicit header variable name
Consolidating the chatty context, repository
Adding authorization enforcement
Renaming question from negative to affirmative

Given that I had an if/else condition, and was asking a negative
question - :policy_unauthorized_for? - I wanted to flip the wording
of the question to be a positive question :policy_authorized_for?

In doing this, I'm hoping to reduce mental dissonance regarding the
policy question.