select code evaluator depending on mode

I have come to rely on running code samples in presentations. Unfortunately currently only evaluating JavaScript code is supported.

This change allows different evaluators to be configured by setting appropriate options. E.g.

```javascript
$.deck.defaults.evaluatorFactoryFor['ruby'] = function(editor, output){
  // retrieve result of execution of code via delegation to a server
  output.html(result);
}
```

Which would enable 'running' Ruby code.