Extract archives to specified directory

[William Friesen <wfriesen gmail com>]

The File Actions plugin currently only provides the action "Extract
Here". I have added an action to choose a specific directory to
extract to, which will check that files do not already exist before
extracting them from the archive. The change is in my github branch
"unpackto" at http://github.com/wfriesen/kupfer/tree/unpackto

A few notes...
-This Action runs asynchronously, but I'm not sure how notifications
should be handled. Currently it just calls uiutils.show_notification
to show that the extraction has completed, and raises an
OperationError when anything goes wrong. Is there a better way to do
this?
-Ideally I'd also implement support for .rar archives, using the
module http://pypi.python.org/pypi/rarfile/ Are there any issues about
including this?