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.
There would be a problem due to curly brace expansion. That's why I've added `set +/-B`, which disables/enables curly brace expansion.