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.
If `$(EUNIT_DIR)` is empty, this has the effect of searching in `.` which means that this variable will contain the beam files of all dependencies too, but also the ones in `ebin`. Example on cowlib:

```EUNIT_MODS = triq_autoexport pdict_statem lock_fsm triq_app triq_expr triq_fsm triq_unicode_tests triq_fsm_stub triq_statem_fsm triq_statem triq_sup triq triq_dom triq_tests triq_app triq_autoexport triq_expr triq_fsm triq_fsm_stub triq_statem triq_statem_fsm triq_sup triq triq_dom cow_spdy cow_mimetypes cow_http_te cow_http cow_multipart cow_qs cow_cookie cow_date cow_http_hd```