Improve plugin singleton pattern

* Make plugin class final
* Add empty constructor method
* Update some inline comments
Better redirect bulk user change. Fixes #2 and #1
Fix displaying admin notices for user bulk change
Remove BP error message handling. Could not find any effective usage of  within the admin
Repair nonce args for bulk user edit nonce field
Explicitly state the removal of sendback query args after user  bulk edit
Adjust verbiage for user bulk edit admin notices
Update some phpDoc headers
Update to first full 1.0.0 version