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 could have used `gen_verbose`. However, I thought that using `eunit_verbose` was a good idea, since it makes it easier to spot what are the EUnit tests you have run. The output looks as follows: `EUNIT mod{1} mod{2} ... mod{n}`.