Pyramid-contextauth

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* Policy checks on each resource lineage and get the first policy it gets.
* Add coverall in after_success of travis config.
* Remove pyramid version constraints.

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* Removing decorator ``authentication_policy``: extension should not
instantiate authentication policy class internally.

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* Registering same context to multiple policies raises a configuration error.
* Unregister old policy when overriding a context with another policy.
* Change register_authentication_policy and authentication_policy signatures.

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* Add introspectables to config for registered authentication policies.
* Rename register_context to register_policy

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* Break backward compatibility as
``ContextBasedAuthenticationPolicy.register_context`` now requires ``config``
instance as first argument.
* Add ``config.register_authentication_policy`` configuration directive which
accepts a list of contexts.
* Use registry adpaters to register policies rather than a dict.
* Add a decorator ``authentication_policy`` to register policies when doing
a config scan.

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* Adjust requirements files and dependencies.