added required icon for feedback icons

Added the option to have a require icon display prior to user interaction to show what fields are required.
This is useful for people that only use Labels for screen readers only, therefore taking advantage of the placeholder attr. for design purposes.

These changes can be viewed and tested in the demo/feedbackicons.html

Please review my code let let me know if there is anything I need to change. 
I manually integrated these changes into a local copy and it was adding the icon to all fields for me.  Maybe it doesn't work right because I don't have all the latest changes, but I fixed it by changing line 292 to check for the notEmpty validator:
```javascript
if (this.options.fields[field].validators["notEmpty"] && this.options.feedbackIcons.required !== null && this.options.feedbackIcons.required !== undefined) {
```