Remove "Keep Sidebar Open" setting.

Just show it in landscape, hide it in portrait.

I could not get UISplitViewController to hide/show the master view immediately
on changing the setting, and I could not get the setting to take effect
consistently on both iOS 5 and iOS 6 without quitting and reopening the app.
This seems like the most sane setting, so let's just leave it.
Don't re-add child view controller in tab view.

Just need to add the view in -loadView, not the child view controller stuff.
Doesn't seem to do any harm, but this feels more right.
Tab view doesn't spuriously load its view.
Animated gif support in image preview

fixes #119
Don't re-download animated gifs.

Just get the data once, then check if we can animate it.
Built-in browser can go back and forth.

Fixes #148.
Fix build instructions in readme.
Don't animate going to Awful thread from settings.

Matches going to threads from other places.