Merge changes and new features suggestion

Hi,

inspired by your message on "bug crushing spree", I'd like to suggest to merge some of my changes (or all, if you like) to mainline.
Most changes concerns Three20Network, probably most notably new TTDownloadRequest/TTDownloadRequestQueue which allows downloads of large files to temporary file and TTURLRequest/TTURLRequestQueue/TTURLRequestDelegate with support of monitoring download progress.
Also some minor changes have been made in Three20UiCommon, adding -isVisibleViewController method, telling if your view controller is currently visible or member of living navigation stack.

Best to all awsome Three20 people
Matthes