Add the ability to delete comment meta fields

Also add support for Bulk Delete comment Meta pro addon
Fix #70
Add a new `array_get_bool` util function
Add the ability to delete User meta fileds

Also add support for Bulk Delete User Meta addon

Fix #87
Update EDD_SL_Plugin_Updater.php to v1.2
Add WordPress 4.0 plugin icons
Add utm_content to addon links
Remove outdated information README file
Increase the height of the right sidebar
Add support for "Delete post by attachment" addon
Fix issue with Google Analytic tracking url
Add extra validation to user input
Update screenshots