Allow to override the repl start function

For node > 0.6 you can now override the repl start function which enables using a non-default repl.

As an example you can now use replpad and get all its extra [features](https://github.com/thlorenz/replpad#features) while debugging your server using `repl-client` as outlined in `examples/replpad.js` and shown below:

![screen shot 2013-08-10 at 1 04 20 pm](https://f.cloud.github.com/assets/192891/942917/3d0d7c68-01e0-11e3-90c7-e33a46ee3852.png)