Added sulu.content.node.save event

Added sulu.content.node.save event. This event will be used e.g. for cache expiring and indexing.

- [x] test coverage
- [x] finish the code
- [ ] gather feedback for my changes


| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| Tests pass?   | yes
| Fixed tickets | none
| Doc           | none