Simplify SGBubbleData interface by collapsing methods.

Replace -contentHeight/-contentWidth with -contentSize
and -totalHeight/-totalWidth with -totalSize.
Simplify abstract factory methods.
Break up SGBubbleTableViewHeaderCell into smaller methods.
Extract a couple of methods in SGBubbleTableViewContentCell.
Remove typingBubble from SGBubbleTableView.

Now use -showTypingBubbleWithDirection: or -hideTypingBubble.
Standardize static inset variable names in SGBubbleData.
Make SGBubbleTypingDirection purely internal to SGBubbleTableViewAdapter.
Eliminate SGBubbleTypingDirection.