Backend updates

db.class.php:
Changed the constructor for PDO_Connect to a more reasonable setup. Also
added default set & get methods.

PDO_Connect function addToTable now requires an object instead of an
array of inputs. Changed this so we actually make use of our objects.
Because of this, I had to change all private class variables to public
class variables.

add.item.php:
Added a category post so we can use this soon.

add.item.form.php:
Wrote a script for categories so it now shows a dropdown list of
categories AND sub categories selected from the database.
Revert "Backend updates"

This reverts commit 0b710b56a1758c4c0c3b3bf692f978c3325b092d.
Revert "Revert "Backend updates""

This reverts commit d83b5da1e3e156ae7566bdbbd1da315ea227f589.