In category listing: load 50 items at a time
admin edit pins: paginate 50 items in category search
edit pins: change to edit with button and image

In order to allow multiple selection of items, the edition of the pin is done now through a button or the pin image.
edit pins: edit only through the button, not image.
edit pins: Select and Unselect all pins
edit pins: Button to delete selected pins
Add an option in admin to remove items from categories
Fix pagination in category selection
Add an option in admin to remove items from categories
Update settings.py

Set SEARCH_PINS to false to prevent site pins from appearing in the search results.
Merge pull request #120 from camilotorresf/fixes

Fixes for pagination and category selection/deselection