Added gallery view controllers to support adding gallery to posts.

GalleryPickerViewController displays a list of available images for a given a post and allows the user to select which images to include in the gallery. A grid view (GalleryPickerGridViewController) will be used when available (iOS 6.0) or a list view (GalleryPickerListViewController) is used otherwise.
GalleryObjectViewController displays a preview of all the images in a gallery.
GalleryScrollViewController is a child controller of GalleryObjectViewController.