added attribute support to the leaflet directive

Unfortunately, some of us more unfortunate souls still need to support ie8. We add the following to support angular-leaflet-directive:

    <script>
        document.createElement('leaflet');
    </script>

This additional code can be eliminated by adding support to the leaflet directive for attributes, and using the leaflet directive in this way:
    
    <div leaflet></div>

**References:**
1. [Angular IE Support](http://docs.angularjs.org/guide/ie)