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.
Regarding "does not solve my concerns that eunit_surefire has more than just the dir option". That's solved by the `.*`, not the `\s*`. I have tested this adding other options (before and after `eunit_surefire`'s `dir` options and this just works. However, if you think dropping this or adding a `EUNIT_REPORT` is the way to go, I am ok with that as well.