Added src publish property to drawer for developer to specify custom icon

Default icon being overridden using css once developer passes src to drawer kind. 
The documentation on StylesheetSupport probably isn't very clear (that's my fault, since I wrote it, poorly). This line and the line above it would actually generate lots of CSS if the src property was changed several times, since each time it changes it's adding to the stylesheet. Instead, you could set the property `stylesheetContent` to the full CSS text, which will then replace any older code from previous changes. Like this:
```javascript
srcChanged: function(old) {
	// Always change the src, even if its set to NULL because we want to be able to restore
	// the initial behavior with `inherit`.
	var src = this.src || 'inherit';
	if (src) {
		if (src != 'none' && src != 'inherit' && src != 'initial') {
			src = 'url(\'' + enyo.path.rewrite(this.src) + '\')';
		}
		// Set the stylesheet to use custom rules
		this.$.activator.set('stylesheetContent', '#' + this.$.activator.id + '.moon-drawers-activator.open:after{background-image:' + src + '; content: ' + (this.handleIconName || '') + '}');
	}
},
```
Does that make sense? Let me know if you have any questions or want any help. :)
Thanks!