Some fixes to the _isArray function.

* Use the native Array.isArray function instead of contrsuctor
  string check
* + Unit test

This is a pretty minor change, mainly wanted to contribute in some way (no matter how small). I don't program a lot of strictly "node" javascript, so I'm very open to feedback on "the right way" to do stuff here.

I also updated the README and added a small section about Contributing, hopefully useful.