Add container interface to get notifications
Add notification classes
Add notification config and ext requirements
Add notification language files
Call notifications when adding or removing from groups
Simplify logic of default setting condition
Adjust lang var name for notifications
Update ext.php removing code duplication
Merge pull request #14 from VSEphpbb/add-notifications

Add notifications support
Add additional test coverage
Fix an error in check_conditions() method
Merge pull request #17 from VSEphpbb/tests

Add additional test coverage and Fixes