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.

Use with Locker, possible in a dedicated stack frame, like this:
// ... code ...
{ // open stack frame
	Locker locker(iu->payload());
	iu->payload()["a"] = "string";
	iu->payload()["b"] = "simultaneouslySentString";
} // close stack frame, sending the update

(by ignoring nonexistent keys)

Also renamed some functions (preserving old names as deprecated aliases)

Also changed constructors of PayloadEntryProxy to protected.

Create doxygen documentation thus:
$ cd docs
$ doxygen ipaacaCpp.Doxyfile

Address mode must be selected explicitly using as_map/as_list, like so:
  for (auto value: iu->payload()["listItem"].as_list()) { .... }
  for (auto kv_pair: iu->payload()["mapItem"].as_map()) { .... }
Generic size() function (reports 0 for non-structured types)

PayloadTypeConversionError thrown if (trimmed) string parse is
incomplete; comparison operators == and != catch this error though.