Add support for test cases

Add check_PROGRAMS and TESTS to Makefile.am, test-driver to gitignore
and LT_INIT([dlopen]) to configure.ac, so eventually, we'll be able to
write tests.

Signed-off-by: Gergely Nagy <algernon@balabit.hu>
trigger-source: New source plugin

This is a very simple plugin, that does nothing more than periodically
create a message out of thin air. The purpose of the plugin is to serve
as a very simple source plugin example, and to aid debugging by making
it easy to generate messages on a timer.

Configuration is simple:

 source s_trigger {
  trigger(trigger-freq(10) # the default
          tags("trigger-happy")
          program-override("trigger")
          host-override("trigger-happy")
  );
 };

Signed-off-by: Gergely Nagy <algernon@balabit.hu>