Refactor library public interface

[paulomarg]

WHY are these changes introduced?

As the code evolved along the project, we've made some changes that made some decisions on how things are exported less optimal. This PR aims to improve our public library, mainly to remove repetition in object layers and to hide session handling functions from Context behind their utils helpers.

WHAT is this pull request doing?

• Changed the client 'namespace' from Shopify.Clients.Rest.RestClient() to Shopify.Clients.Rest()
• Changed the session 'namespace' from Shopify.Auth.Session() to Shopify.Session()
• Changed the OAuth 'namespace' from Shopify.Auth.OAuth to Shopify.Auth
• Moved the Context functions loadSession, storeSession and deleteSession into a new folder auth/utils which is not exported, which forces apps to go through Shopify.Utils.loadCurrentSession(), Shopify.Utils.deleteCurrentSession() and a new Shopify.Utils.storeSession() which hold the logic for id management.

Type of change

• Patch: Bug (non-breaking change which fixes an issue)
• Minor: New feature (non-breaking change which adds functionality)
• Major: Breaking change (fix or feature that would cause existing functionality to not work as expected)

[paulomarg]

Thanks @thecodepixi. I'm also cc-ing the rest of the team here, I'm keen on getting as much feedback as I can on this one :)