proposed strategy for integrating with pyramid_authstack

[tripsolutions]

In relation to issue #53, this would be a first draft proposal for enabling integration with pyramid_authstack.

• rewritten _configure from init.py to be consumed externally
• made the registration optional
• removed request property authentication_policy and turned _request_claims and _request_create_token into closures instead
• added create_jwt_cookie_authentication_policy with the same function as create_jwt_authentication_policy
• added example in the documentation

[crooksey]

@wichert any thoughts on this?