Uses repository pattern to fetch ShopifyAPI::Session

Also implemented a simple in memory storage adapter that associates
the ShopifyAPI::Session with a UUID. It's not really a solution for
production environments but does provide a starting point. Basically
what you want is a really thin adapter that makes it easy to pull the
associated sessions from your database.

If you are using ActiveRecord, the only method you'd need to implement
is store on the relevant Shop model for your application
Put the session fetching in the right place
Improves the documentation and makes some changes to the API
cleanup some stuff
retrieve not find
session[:shopify] is no longer the truth.

Need to verify the object associated with the session is valid
Merge pull request #92 from Shopify/shopify_session_repository

Uses repository pattern to fetch ShopifyAPI::Session