Addign no auto write body option

Some times you only need to get the rendered result (html) to do differents thinks... for example... if you need to send an email or create a ajax content... for that reason I add a default configuration named writeResp (write reponse). If this configuration is true, it writes the body response automatically.. if it is false, the context function return the html generated.

Example:
```
var html = yield this.render('admin/index',{layout:'layouts/admin',writeResp:false})
```