Feature/retry media upload

Implement retry functionality for media uploads that fails.

Solves: 
 * wordpress-mobile/WordPress-iOS-Editor#348
 * wordpress-mobile/WordPress-iOS-Editor#361

cc @bummytime @diegoreymendez 
@SergioEstevao - Didn't see the `!`... but in that case I'd just simplify `!(progress.totalUnitCount == 0)` to `progress.totalUnitCount != 0`