Transclude support for leaflet-directive

With this patch, in addition to current argument-style html definitions you can also use transcluded directives  for example these two are now indentical:

```html
  <leaflet marker="markers"></leaflet>

  <leaflet>
    <markers="markers"></leaflet>
  </leaflet>
```

This allows much more customizable behaviour of leaflet directive and custom made directives.