Add eunit support

Introduce a new make target – 'eunit' (looks up in src/ and in test/)
Introduce a new parameter – EUNIT_OPTS

Everything is being compiled into ./.eunit folder, just to keep this stuff isolated.

Maybe it's actually better to replace EUNIT_OPTS with EUNIT_VERBOSE, because so far, the only way to use EUNIT_OPTS is:

```shell
EUNIT_OPTS = [verbose]
```

Thoughts?