File events

Design and implementation of file events based on an event bus
- Branch is based on dev, so merging into dev is probably the way to go at the moment (merge with the master later)

- Library for the event bus:  MBassador, small, but useful (added as maven dependency)

- The reason why I used an additional library is that it would get quite messy the more events we will implement if the plain old observer pattern is used. 
I tried to implement it with the observer pattern first, but it leads to either pass around the observers or pass down high level managers that do the event handling. The bus decouples the senders and receivers of events as a sender only needs to know the bus and a receiver only needs to subscribe particular events without knowing the actual senders. Also, there is support for async event delivery which may be useful depending on some use case. 

- I was not able to prevent changing constructors etc. in some classes as some of them are very "low level" in the hierarchy (e.g. the download tasks etc.). Another way would be do include the bus in the process framework, but I did not want to change the API (yet) just for file events.

- Unit tests: core.events.FileEventsTest (test classes for operations add/delete/remove)

See also commit message. 

Open issues
- documentation, how to use/register for events
- more events as needed, e.g. related to sharing
- end-to-end testing with the H2HNode and the H2HManagers (current tests use the NetworkManager directly)