Support for [UIFont preferredFontForTextStyle:]

[dzindra]

This PR adds ability to style font properties with values of [UIFont preferredFontForTextStyle:].

To use it, instead of font name and size, use one of the following as font name:

• body for UIFontTextStyleBody
• caption1 for UIFontTextStyleCaption1
• caption2 for UIFontTextStyleCaption1
• footnote for UIFontTextStyleFootnote
• headline for UIFontTextStyleHeadline
• subheadline for UIFontTextStyleSubheadline

So instead

UILabel {
  font: "Helvetica" 14;
}

you use

UILabel {
  font: body;
}

On iOS6, the fonts will not be recognized, so you should use media queries.

[cloudkite]

Thanks!