(bug 47086) improve image caching

Caches thumb files explicitly in local filesystem so we can
load them synchronously during scrolling. This reduces annoying
reload delays from relying on NSURLConnection, which always
returns asynchronously.
bug 47085: Add cell index numbers to match Android

These sometimes get out of sync when adding new items, but they clear
up when scrolling around.
Merge pull request #39 from montehurd/smallScreens

Improved layout on 3.5 inch screens