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 
I don't agree with this, returning immediately makes the code a lot easier to read and maintain in my opinion. Several levels of indentation and options makes it harder to read and follow.