Change class prefix from UI/NS to SG.

UI and NS are reserved class prefixes.
Rename SGBubbleTableViewCell -> SGBubbleTableViewContentCell.
Rename SGBubbleHeaderTableViewCell -> SGBubbleTableViewHeaderCell.
Rename SGBubbleTypingTableViewCell -> SGBubbleTableViewTypingCell.
Rename NSBubbleType -> SGBubbleDirection.

Also rename type -> direction and enum states:

BubbleTypeMine -> SGBubbleDirectionRight
BubbleTypeSomeoneElse -> SGBubbleDirectionLeft