Add support for popups on paths

Hi!
Just added support for popups defined on paths. We are just working on a project where we visualize the layout of a huge computer network and showing information about connections was supposed to be good idea. Thus, I was already happy to see that someone integrated label support. But bringing up a popup offers more options (like embedding links to other systems).

![leaflet-angular-paths](https://f.cloud.github.com/assets/6880844/2371720/d6c12820-a82d-11e3-9b36-6292360baa5a.png)