Timer has timeout period and a handler; imagine possibility to have many concurrent timers.
------------- license FreeBSD license
------------- how it works Check test/test.cpp.
------------- TODOs
- error handling philosophy
- error return codes
- allow / disallow exceptions
- added support for milliseconds
- many listeners to a single timer
- listener is provided to timer in constructor
- supported multi-threading level explained
- periodic emission of events
- executor count is minimized over time, if they are not needed
- etc.