add back wikirate_tag.rb and 

1. add back wikirate_tag.rb
2. add default html tooltip for the clipboard
3. change readme from text to md format
4. add config file for link_thumbnailer to repo

this is good, I'll go ahead and merge it in.

note that if you do create! (with the bang), it will raise an error if there is a problem.  if you do Card.create, it will add error messages to the card (which you can then handle).  In general, it's preferable to handle these things explicitly, but this is fine for now.