32 admin portal setup

## 32-admin-portal-setup
* Admin portal will be `admin` namespace (routes change).
* Create a new layout `layouts/admin.slim `
* Create `admin/base` controller which will be an abstract controller for other admin controllers to inherit from
* Tell the `admin/base_controller` to use the `admin` layout
* Implement HTTP Basic authentication in `admin/base_controller` : http://api.rubyonrails.org/classes/ActionController/HttpAuthentication/Basic.html
* Generate a admin/dashboard controller which will be the root of the /admin namespace