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.
@jj1bdx made the great suggestion that using `-s erlang halt` here instead is much faster. I did not try to see if it works well with the rest of your command though.