LoggerAppenderMail: added dependency injection

The Mailer object can be injected, this makes the class testable and
should fix problems with TravisCI.

Signed-off-by: Ivan Habunek <ihabunek@apache.org>
Removed LoggerAppenderMailEvent

All the functionality is available in LoggerAppenderMail.

Signed-off-by: Ivan Habunek <ihabunek@apache.org>
Added recent changes to changelog.

Signed-off-by: Ivan Habunek <ihabunek@apache.org>