[PLUGIN] difftools updated

[Stephen Ward]

I've updated bzr_difftools with the following fixes:

* Import cleanup to fix the interaction with lazy_import in bzr 0.12 
  (thanks to John for doing much of the work)

* Added an optional pause before cleaning up temporary files, which should
  make MacOS 'opendiff' happier.

The tarball and branch have both been updated, and can be accessed from the 
Plugin Registry as usual.

-- Steve