- Links use the same datastructure as Payload and Metadata.
- Payload, Metadata, and Links are now JSON-based from the first level.
- There are now two messages IUUpdate and IUUpdateRequest, the former
  to be sent out by OutputBuffers (always succeeding), the latter to be
  sent out by InputBuffers.
- Made more fields optional.

Disabled removal of retracted IUs from InputBuffer to
match the Python version - since it is an item of discussion
that will be resolved in the next version anyway.

They are torn down on program exit (retracting all live IUs)

- OutputBuffer retracts all remaining live IUs on destruction
- sends correct event type now
- fixed reference error for calling handlers
- setters properly check for retracted flag
- also added some missing checks for committed flag

* Exceptions when trying to modify retracted IUs.
* Removal of IU also retracts it.

- OutputBuffer retracts all remaining live IUs on destruction
- sends correct event type now
- fixed reference error for calling handlers
- setters properly check for retracted flag
- also added some missing checks for committed flag

* Filtering by IU event type.
* Description of sniffer arguments after startup now prettier.

Also removed stray cout lines

Manually integrated the following changes from b8466a (master 2015-11-13)

- Blackboard classes by hvanwelbergen
- Blackboard test by hvanwelbergen
- Payload merge fix by hvanwelbergen
- rsb Buffer fix (by myself) copied but inactive (not needed atm?)

Please refer to the logs of the legacy-str branch for a full history
of the respective changes.