Refactor controller spike

I did some refactoring of your spike:
- added authentication module for all authentication logic
- added instagram_oauth module for all oauth logic
- landing page is now instagram#index
- authenticated users will be redirected to instagram#show
- only create a user if they don't exist in the database
- added a logout link
- removed unused actions/code

Important: this is untested code and therefore not safe to push to master/production. This is a spike code. I think your spike is over, this branch served it's purpose. Now cut a new branch and start developing the feature. Start by writing tests (models, controllers and features). There is only 1 way to know that code works, we test it. 

I never ever ever ever submit code that's not tested.