add support for images via saving them to the local FS when sharing

This pull adds functionality for `TTOpenInAppActivity` to handle `UIImage` `activityItems` by saving them and using the URL on the fly.

This is necessary because of apple's `AND` matching on extensions - if we want to support image handlers, we can't send any activities that are of type NSURL, as most iOS 8 share extensions will ignore them. (details here: https://github.com/tumblr/ios-extension-issues/issues/5).

Not sure if this is too specific to go in the TTOpenIn core, but we needed it and a PR was requested, so I figured we could have a discussion on if it's appropriate.