Background concurrent image fetch w/dynamic priority q

Image downloading was locking up if you quickly scrolled through a large set of images. Too many NSURLConnection threads were spawned.

Now, based on how long downloads are taking, a variable number of concurrent download queues are created then sent to a dedicated NSOperationsQueue which fires them off on background threads.

Download operations in the queue are also now dynamically prioritized to favor images for cells which are presently on-screen. Interface seems much more responsive - things start appearing on-screen much faster when dealing with large image sets.

In addition to new FetchImageOperation there is new SpeedGovernor object encapsulating download speed heuristics.
Merge remote-tracking branch 'monte/over500Uploads'

Conflicts:
	Commons-iOS.xcodeproj/project.pbxproj