Added "Copy" menu to copy contents of chat bubbles to UIPasteboard

I appropriated some code from https://github.com/ardalahmet/CopyableCell to add Copy functionality. 

As of iOS5 there is a simple technique to do this using UITableView delegates but the resulting copy menu is centered across the entire screen. Using this older method I have the copy menu centered over the text bubble.

If the NSBubbleData member contains a UILabel it is copied as text. If it contains a UIImage or UIImageView it is copied as an image.