Convert appledoc to use ARC

This resolves Issue #439 and also Issue #463 (and possibly others.)

Right in the middle I attempted an upgrade to a more recent version of GRMustache (which I thought was necessary) but turned out to be problematic. If there were changes made to the Mustache spec, or customization of Mustache used for appledoc, that would explain it.

Confirmed that AppledocTest passes, and ran a sample generation of documents on appledoc itself. Both worked perfectly.