Closes #84: Globally enable $http cache for CartoAPIService

This seems to remove most of the sluggishness of the app.
Best guess is that we were sending too many requests from
a single IP and getting throttled. With the cache in place
the app seems to function as normal and quite a bit snappier.
Add overlay loading div when transitioning map > compare|detail

If the transition to detail or compare takes more than overlayTriggerMillis
then overlay the loading spinner. This should probably be styled further.

I was only able to get this overlay to appear a few times after
enabling the $http cache option in the previous commit.
Merge pull request #99 from CloudNiner/feature/investigate-transition-slow

Feature/investigate transition slow