Perform callbacks in the Torrent class using GCD.

git-svn-id: svn://svn.transmissionbt.com/Transmission/trunk@14118 f4695dd4-2c0a-0410-b89c-da849a56a58e
Simplify the torrent complete callback method.

git-svn-id: svn://svn.transmissionbt.com/Transmission/trunk@14119 f4695dd4-2c0a-0410-b89c-da849a56a58e