Added ability to set defaultEngine for the express app.

Will default to js
//usage
var server = new Server({defaultEngine: 'coffee'});

I was having difficulty setting coffee as the default view engine for express.

It would raise an exception saying "500 Error: Failed to lookup view …" When I changed this to "coffee" it worked. This allows the "view engine" to be set to a custom value.

I tried adding `app.set('view engine', 'coffee');` in my code before and/or after the creation of the rendr server to no avail.