Fixes to credential input and implemented the ability to stop the upload operation

I changed the behaviour of **SettingsViewController** so that it doesn't have to re-validate the credentials if they weren't changed, which speeds up getting in and out of the view.

Also, I implemented the ability to cancel the upload operation after it has begun. I changed the *Upload* bar button item in **TableViewController** to toggle between it and a *Cancel* button so that it will stop the upload and reset the progress on the active **FileUpload**.