Work in progress for #132: Added message preview to iOS example.

* Operations are not yet cancelled at any point.
* There is a visible delay when the detail text of table cells are
  updated after a delay as cells are re-used.
Merge remote-tracking branch 'upstream/master' into message-preview-in-examples
Merge remote-tracking branch 'upstream/master' into message-preview-in-examples

Conflicts:
	example/ios/iOS UI Test/iOS UI Test/en.lproj/MainStoryboard.storyboard
Restored table view cell style to subtitle.
* Lost in the merge conflict.
Associating message rendering operation with table view cell.
* Added custom table view cell.
* Cancelling operation on cell reuse.
Merge remote-tracking branch 'upstream/master' into message-preview-in-examples
Setting the rendering operation to nil when the complete.
Renamed block parameter.
Cancelling ay previous rendering operation ASAP.
Clearing previous cell subtitle on reuse.
Added prepareForReuse
* Moved cancellation of rendering operation and resetting of subtitle.
Registered a class for cell reuse.
Merge pull request #191 from paulyoung/message-preview-in-examples

Add message preview to iOS example.