Fix error with default icon image path

[tonimc]

If you try to use leaflet-map inside a component, the component fails because leaflet is not able to get the default icon image path due to leaftlet-map suppose that you are imported the component main document.

Using polymer component method resolveUrl (https://www.polymer-project.org/1.0/docs/api/Polymer.Base#method-resolveUrl) you can get the URL of a component and use it to get the correct icon default URL.

[fegan104]

This just saved me so much time; thank you!