Modification for publishing events to indicate artifact deployment status.

- The modification were done, due to the requirements of WSO2-AppFactory product.
- Introduced new two events to indicate 'artifact deployment started' and 'artifact deployment completed'  points.
- Cartridge agent was modified to get updated artifacts with their last modified time in the repository.
- Cartridge agent was modified to publish new events to message broker.
- Messaging component was modified to process new two event types. (Introduced few new classes to match the existing pattern of the code)
Perhaps, it's a good idea to replace "GitAPIException" with some meaningful name.