added transclude support for customization

Hi,
this addresses #322

this enables you to create new directives with an isolated scope  but able to use the leaflet-directive. so
by the following one can create its own directive `myLeaf` and require `^leaflet` to get acces to leaflets controller.

```html
<leaflet defaults="defaults" markers="markers" center="center">
    <div my-leaf="myCustomLeaf" />
</leaflet>
```

since im not complete sure about the functionality behind the whole leaflet-directive package i didnt touch the "package plugins" so directives like center, bounds, markers ... still have to be attributes of the same leaflet element