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

Please review @maartenvg @mtaher @jeromecornet for the code and stuff
Please provide a security review @HoLyVieR just to be sure I'm not being a goof anywhere

This is an improvement based on a conversation with @davefp and seeing Pull Request #90 by @choonkeat

Fixes #85 
Should not it be `def self.retrieve(id)`?