New Plugin updater. Fixes #1332

This is seriously cool. and fixes #1332 ;-)

It adds a new modal window for installing plugins with the following features:

* Pick and choose which plugins you want to update
* View changes
* View old/new version number
* Fancy sliding/resizing (OK, that's not a feature) :P
* scope for much more

It could probably do with a bit of tidying up round the edges, so I thought I'd open a pull for discussion.

To test this out, you need to have some 'old' plugins on your system that you can update. Here's a script to install old versions of Safari, Terminal and Contacts (assuming you have the latest). Run it then restart QS and click 'check now' for updates:

```bash
#!/bin/sh

cd "$HOME/Library/Application Support/Quicksilver/PlugIns/"
# remove new plugins
for PLUGIN in "com.blacktree.Quicksilver.QSSafariPlugIn.137.qsplugin" "com.blacktree.Quicksilver.QSTerminalPlugIn.139.qsplugin" "com.blacktree.Quicksilver.QSAddressBookPlugIn.171.qsplugin"
do
    rm -rf $PLUGIN
done
# get older plugins
for PLUGIN in "com.blacktree.Quicksilver.QSSafariPlugIn__310.qspkg" "com.blacktree.Quicksilver.QSTerminalPlugIn__295.qspkg" "com.blacktree.Quicksilver.QSAddressBookPlugIn__366.qspkg"
do
    wget "http://qsapp.com/qs0/plugins/files/$PLUGIN"
    ditto -x -rsrc $PLUGIN .
    rm $PLUGIN
done
```
![Screen Shot 2013-01-27 at 12 11 30](https://f.cloud.github.com/assets/150431/100848/d70f5304-687a-11e2-8921-e6a8684f8bac.png)