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.
Looks like you have a whitespace mess here.

I would like to have shared building of eunit and common_test *if it makes sense*. Does it?