Set Parse-RACExtensions submodule
Fix issue in activity view.

Cherry-picked from ParsePlatform/AnyWall@0acd62f36bf3de37caa0718edd91ab9cb25ee961
Explain fork in README
Take Parse keys from env
Cleanup for RAC conversion
Move and inject filterDistance & currentLocation

Moved these properties from the app delegate into the root view
controller, `PAWWallViewController`. From here, these properties are
reactively propogated to child view controllers. Dependency injection,
if you prefer.
Observe authoring of Posts
Write distinct search radius updates to user defaults
Streamline radius overlay mechanics
Lift -updatePostsForLocation:withNearbyDistance:
Lift -queryForAllPostsNearLocation:
Update map geometry when tracking changes

This updates the map center or region when either the current location
or the filter distance respectively change. But, the map is only
updated when tracking the current location.

Note, this commit actually removes some behavior. In my defense, there
was a comment that said:

"// Just zoom to the new search radius (or maybe don't even do that?)"

I agree with the doubt, so I removed that behavior.
Query nearby posts when location changes
Use signalForSelector over map view delegate methods
Use RACified CLLocationManager