Add Coveralls configuration

@nylen you just need to enable coveralls for the request project on https://coveralls.io, other than that technically it should work. I tried it on one of my projects with _mocha_ instead of _tape_ and it seems to be fine.

Also you can try this line out

```bash
./node_modules/.bin/istanbul cover ./node_modules/tape/bin/tape tests/test-*.js
```

This will generate a nice html output in `coverage/lcov-report/index.html`, so you can check the output locally as well.