project modernisation
Add catalog entries for adding shelf/clipboard items to the catalog

Fixes quicksilver/quicksilver#536
Add documentation
Improve docs layout
Disabled the new shelf and clipboard contents catalog entries by default
Invalidate the shelf contents entry when its contents are changed
Make the shelf plugin recommended
Remove a deprecated method
Merge pull request #8 from quicksilver/stuff

Stuff. Fixes quicksilver/quicksilver#536