Add view with causes

* Add a causes view using same incidents as main view
* Create new GeoJSON with marker color based on cause
* Add legend identifying causes
I don't know what i would do exactly, but I'm thinking I might create something that generates that finds the correct presenters.

So something like,
```ruby
class CauseMarker
  CAUSES=[
    Environmental,
    RoadCondition,
    UnsafeBehavior,
    ImproperBehavior,
    Unspecified
  ]

def self.build(incident)
  INCIDENTS.map { |cause_class|
    incident_class.new(incident.cause)
  }.select?(:valid?) || NoCause.new
end
```
Then in each of those you would specify the color.
 
Seems like a bit much to me