util/Signal:  added `clear` method to remove all callbacks
util/WorkerObject: added `get_io_service` method

...you can probably guess what it does, no?  Well, it's documented if
you really can't...
util/Signal: allow setting of the io_service pointer post-construction
util/Signal: fix a typo introduced in @51d3ae9
comms/Message...: remove useless "Type" suffixes from template params
Added Doxygen configuration.

To generate API documentation in doc/html, run `doxygen doc/Doxyfile` from
the project's root directory.
util/{Signal[Action],SharedQueue}: small documentation improvements
util/Signal: fix incorrect name for `set_io_service` in implementation