SectionedDocumentViewController now uses white-on-black section titles

the default colors are black-on-gray, which has insufficient contrast;
the new white-on-black colors also fit better into the new iOS 7 app
look (e.g. on iPad the two board position table views also use
white-on-black section titles).
set contentsScale property on all CALayer objects to support Retina displays (#205)

the contentsScale property, which exists both on UIView and CALayer, is
an important property for properly supporting Retina displays. UIKit
automatically sets the property to its correct value on UIView objects,
but does not do so on CALayer objects - there we need to manually set
the property.

after this commit, coordinate labels are now displayed crisp and sharp
on Retina displays.
move numbers and the next move symbol are now drawn crisp on Retina displays (#205)

the fix is to draw directly to the CALayer graphics context, instead of
drawing into a newly created CGLayer and then "pasting" that CGLayer
onto the CALayer.