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.
-DTEST should be -DTEST=1 (otherwise issues may occur depending on how you test for TEST). Also please add -DEXTRA=1.