script hash rails integration PoC

Application demonstrating this: https://github.com/oreoshake/script_hash_test
Video demo: https://www.youtube.com/embed/Bc2hvziTRxg

Two pieces:
* Rake task to find all inline script, compute the script-hash value, store in config/script_hash.yml

AND 

* Contains new before_filter to subscribe to ActiveSupport::Notifications events (rendering partials/templates)
* All rendered templates write to an env value that contains all hashes for each rendered template/partial
* Middleware then (stupidly) does a find/replace or adds a directive containing the script hash values

Based on "simple" script hash proposal.
Need to verify hashed values with various methods.

This makes me think the before_filters should create a ContentSecurityPolicy object and build the entire header in the middleware.
It should also be sha1. Only sha1 is supported for now afaik.

Please do submit a PR if you have the time, including your other changes. Or submit a fresh PR :)