Further reduction in fast-scroll jitter

Was able use dispatch_async to make it so even the loading of cached images doesn't gum up the main thread.
Made the other places which call fetchThumbnails use dispatch_async

This should smooth details and full-screen pages a bit.
FetchImageOperations can now report the progress of their downloads

Also made the interface show the download progression by adjusting the shading of the file name info box.
Merge pull request #65 from montehurd/jitter

More jitter reduction
Download progress indicator reflects previous progress

If image cell went offscree then back onscreen it wouldn't reflect previous download progress until it receive a new chunk. Now it remembers previous progress.
Bug 48040: Check for filename conflicts before upload

Checks eg 'Foo.jpg', 'Foo 2.jpg', 'Foo 3.jpg' etc in sequence,
and uploads using the first title that is found to be unused
by an image on the target wiki.

Still possible to get conflicts from *during* the upload, but
they'll be much less likely.

https://bugzilla.wikimedia.org/show_bug.cgi?id=48040
https://trello.com/card/ios-handle-filename-conflicts-transparently/51830a8104d981c70c0012ce/29
Merge pull request #66 from brion/namecheck

Bug 48040: Check for filename conflicts before upload
Merged thumb download progress indication on to latest
Merge pull request #67 from montehurd/downloadProgress

Download progress indicator for thumbnails