Fixing pages/setup.php writing style

- Use single instead of double quote for get-text which doesn’t use any
variable
- The correct text-domain should be wp-ig instead of wp_ig. Dashes, not
underscores
Adding category & post type selector at setup page
Adding WP_IG_Content

the class which handles content-related stuff
Adding content deleting mechanism

A “delete all” page / action
Registering "mention" taxonomy
Early working version of importing feature

pages/import.php still need lots of refactoring. Still looks ugly, but
it works