code: def _build_filepath_for_phantomcss(filepath):
      
      try:
          if os.path.exists(filepath):
              new_root = '.'.join((os.path.splitext(filepath)[0], 'diff'))
              ext = os.path.splitext(filepath)[1]
              diff_filepath = ''.join((new_root, ext))
              if os.path.exists(diff_filepath):
                  print 'removing stale diff: {0}'.format(diff_filepath)
                  os.remove(diff_filepath)
              return diff_filepath
          else:
              return filepath
      except OSError, e:
          print e
docs: Prepare screenshot filename for use with phantomcss.
          ie, append 'diff' to the end of the file if a baseline exists