Fixes and improvements.

Fix: **pagination**, first element must be calculated before updating last element.

Partial fix: <b>`$wpdb->prepare()`</b> - I'm not sure how to include order and orderby as `prepare()` arguments. The sortable columns only work if I do it like in this pull request.

Improvements: added 4 **filter hooks** to deal with Columns, Sortable Columns, Post Types and Per Page value.

I donnow how versioning and collaboration credits are done when forking/pulling :)