Change minimum requirement to WordPress v3.3

This Plugin uses WP_Screen which is available only from WordPress v3.3
and above.

Fix #13
Fix compatibility issues with "The event calendar" Plugin

The event calendar Plugin rewrites query variables using `pre_get_posts`
hook, which fails the query issued by Bulk Delete Plugin.

This commit adds an hack, which disables and enables the filter if that
Plugin is installed. It is not ideal, but it gets the work done.

Fix #14
Regenerate pot file for translation