Added ability to set defaultEngine for the express app.

Will default to js
//usage
var server = new Server({defaultEngine: 'coffee'});
Updated pull request based on good ideas expressed by dominikkrejcik
in https://github.com/airbnb/rendr/pull/95
Merge pull request #132 from givingstage/default_engine_for_express

Added ability to set defaultEngine for the express app.