Add proper text formatting for messages

[viktorstrate]

This fixes #63. Instead of rendering the raw markdown body of a message, with some limited formatting, instead convert the formattedBody HTML attribute into an NSAttributedString that is rendered in an NSTextField.

Todos

• Fix issues with calculating the proper height of timeline rows.
• Tweak the formatting options and spacing.
• Make it work with custom font sizes.

[viktorstrate]

Although the formatting is not always perfect, this is mostly a limitation of the ZMarkupParser library. Eventually we would probably want a custom HTML parser to NSAttributedString, such that we have complete control of the rendering.