Drop uninteresting webkit tests.
Refactor and test build_policy.

In prep for overriding decorators, refactor build_policy and add tests
for each individual settings.
Enable updating or replacing rules from settings.

Prep work for decorators.
Make tests a module, add middleware tests.
Make middleware use config, update, replace.

Pull optional attributes off the response and pass them to build_policy.
Add policy-modifying decorators.

Fix #11 by adding @csp_update. Fix #12 by adding @csp_replace.