v2.3.0

• #29 Add "hold to preview" feature (Finally!)
• #32 Fix showing "No photos" view issue even there are photos in album
• #35 Fix showing supplementary view issue when there are no assets in the album
• #38 Add shouldShowAsset delegate method
• #39 Add showsNumberOfAssets property
• #40 Change the picker to a container view controller rather than a navigation controller.
• #41 Add "show default content" feature
• Make assets group cell to opaque to increase scrolling performance
• Add accessibility label to the auxiliary view