Fixed som bugs when updating markers model

Hi,
I found some bugs when trying to update markers in real-time. I've added an example, you can test the changes switching the script location.

Problems I tryed to solve are:
- No testing of possible bad 'numbers' in lat and lng
- Popup contents was updated to the full data object, not only the message.
- If popup is binded (i.e. there was a previous message - leaflet does not provide a function to query if a marker has a binded popup) update its content, no bind again. Binding again leaves previous opened popups in the map. Also if there was no previous message, then I bind the new popup.

I'm pretty new in angular and also this is my first contribution ever using github. I'll be waiting for your comments and istructions on how to proceed.
Jaume.