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 
It's the global progress of all media uploads that are happening. You can have 4 uploads going that are in the mediaInProgress array and this progress is the master progress of all then.