board zooming is now also available on iPad devices (#16)

initially iPad devices were thought to be large enough so that zooming would not
be necessary for them; this decision had to be overthrown after remembering the
iPad Mini's smaller screen size
PlayViewLayerDelegateBase class is now responsible for disabling animations
GoVertex has 2 new properties to access the vertex compounds individually
first draft for displaying coordinate label (#12)

this first draft implementation is basic but should be functional; tests
were made for a 7x7 board on the iPhone simulator; stuff that should
already work
- new user preference
- coordinates are displayed as an overlay of the Play view so that they
  are always visible even if the Play view is zoomed in and scrolled
  (this way of displaying coordinates was stolen from the app
  Panda-Tetsuki)
- coordinates have a drop shadow so that they are visible even if they
  are overlaid over a stone that has the same color as the label text

the following still needs to be done
- use variable font size depending on board size
- only reserve space for one strip of coordinate labels (or make it
  a preference whether the user wants to have 1 or 2 strips)
- measure performance and memory usage
use variable coordinate label font size (#12)
reserve space for only one strip of coordinate labels (#12)
eliminate boardOuterMargin user defaults property
anti-aliasing fix for fractional view size after zoom

without this fix, grid lines are often drawn with anti-aliasing after a zoom
operation completes; even after this fix, anti-aliasing still sometimes
occurs, but less often; the reason for the problem is currently unknown