Move util functions to a separate file
Remove release notes from the main plugin file

This information is present in readme file. There is no reason to
duplicate this information in two places
Rename display_setting_page to display_posts_page
Move static functions to separate classes

Fix #18
Add the ability to delete posts based on custom field
Add information about delete by custom field addon
Bump up version number to 4.1 for release
Regenerate pot file for v4.1