Retry media upload - Take 2

Implement retry functionality for media uploads that fails.

Solves: 
 * wordpress-mobile/WordPress-iOS-Editor#348
 * wordpress-mobile/WordPress-iOS-Editor#361

cc @bummytime @diegoreymendez 
OK dumb question here :grinning:....for failed media wouldn't we need to check for something else besides the count of the media in progress? Like ```!self.isMediaUploading && self.mediaInProgress.count > 0``` or something.