EUnit plugin

Running `make tests` will now also run EUnit tests.

Users can set different `ERLC` and `EUNIT` options by providing the `EUNIT_ERLC_OPTS` and `EUNIT_OPTS` variables, respectively.
This part seems brittle at best. It'll break as soon as one puts a space in the wrong place, or add an option before/after `dir` (surefire also takes a "verbose" option). I would advise remove this (and the related one below) and instead documenting this case properly in the README.

And while at it you might want to document the plugin in the README, I almost forgot. :-)