Upgrade to Xcode 5.1, and fix a bunch of 32/64-bit problems.
Add the following changesets:

commit d1f06b1ede56ceaee4309076abd7d22fb3d82b71
Author: Marc Brown <marc@creativemess.com>
Date:   Sun Mar 31 22:33:58 2013 -0400

    Replace isa getter with object_getClass(object)

commit 18b19767170bceb7c3d350142f30e6afce961816
Author: Marc Brown <marc@creativemess.com>
Date:   Sun Mar 31 22:31:55 2013 -0400

    Replace isa setter with object_setClass(object, Class)

commit cd601ac912c0c14e454da492d6e9ae9c1ec54b51
Author: Marc Brown <marc@creativemess.com>
Date:   Sun Mar 31 22:27:51 2013 -0400

    Explicitly cast format arguments as unsigned long
Remove the workaround for a bug in OS X 10.7, and the big comment since
none of it is applicable any more.

This removes code that Xcode 5.1 now flags as "highly discouraged".