Add 'Categories' link to plugin submenu on the admin
Add `setup_views` method to load the view for each page on the admin menu
Add categories view
Fix comment
List all the categories on the database
Remove extra header
Fix table names

Change hardcoded table names to correct handle the table prefix used by
the WP installation
Remove `init` method in favor of `admin_init` hook
Remove unnecessary include
Save category when submitted correctly
Merge branch 'feature/categories' into develop

* feature/categories:
  Save category when submitted correctly
  Remove unnecessary include
  Remove `init` method in favor of `admin_init` hook
  Fix table names
  Remove extra header
  List all the categories on the database
  Fix comment
  Add categories view
  Add `setup_views` method to load the view for each page on the admin menu
  Add 'Categories' link to plugin submenu on the admin