Expose Mincer once instantiated so Engine Options can be set a runtime

Mincer sets its CoffeeScript Engine to "bare" by default (against the CoffeeScript default). By default I mean when compiling .coffee with coffee -c the top level function safety wrapper is applied to the file.

So exposing Mincer in this commit allows this option to be changed back to the default at runtime when using connect-mincer.

```
var ConnectMincer = require('connect-mincer');
var connectMincer = new ConnectMincer({
   /* */
});

connectMincer.Mincer.CoffeeEngine.setOptions({bare: false});
```