Add Mac OS X support

* Add support for building a .app package version of Trelby
  using py2app. Use the build_mac.sh script to create the
  necessary files and call setup.py with the necessary
  arguments.
* Set the appropriate font to use for Mac OS X.
* Work around text selection/alignment issues. (Still not
  perfect, but much better than before.)
* Allow usage of newer wxPython version. Work around error
  dialog bug in Mac wxPython.