Add progress report for media uploading.

Use NSProgress to report uploading progress.
Add NSProgress to interface of MediaService.
Fix error callback calls and make them use current queue instead of main one.
Improved documentation of MediaService.
Better fix for relative URLs
Cleans up the editor's Blog object handling.
Removed an unnecessary file.  Code cleanup.
Add progress reporting for setting featured image.
Only increase size by 1 to avoid show incorrect size of file.
Add progress reporting view controller to legacy editor.
Replace cancel button for done button.
Several improvements to our new editor.
Several improvements to the code to handle saving posts.
Fixes some issues with the pages editor.
Fixed a logic error saving a removed post's context.
Merged from release/4.6
Add a progress view on navbar to show progress on uploading media.
Improve progress reporting by only incrementing when all file is completed.
Fix progress report in XMLRPC code.
Improve progress report and allow to stop single uploads.
Add a cancel all button for media upload.