Add essential information to map

![untitled](https://cloud.githubusercontent.com/assets/6946910/5389231/062ceb36-80c7-11e4-8844-3e420be66062.png)
![untitled2](https://cloud.githubusercontent.com/assets/6946910/5389232/0960089c-80c7-11e4-8888-abd1f2df0ce3.png)

* Add cause and vehicle to marker popups
* Add map key explaining markers
* Add legend showing dates and incident count
* Style the above