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.
Yes, adding a lot of `\s*` was not my point and does not solve my concerns that `eunit_surefire` has more than just the `dir` option. This whole regexp matching sounds very dangerous.

I think @dch's suggestions are both sound. I would consider dropping if there doesn't seem to be anyone needing a different directory at this time, otherwise going with the variable.