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.
I think I took this from the `ct` plugin. There is no need to re-compile `ebin` with `EUNIT_ERLC_OPTS` in `build-eunit`, because this has already been done in the `eunit` target.