don't crash current request on missing mf2 properties. fixes #29
use mf2 u-url property when available. fixes #23

adds a new links.url db column, so @aaronpk, you'll need to run `rake db:migrate` before deploying this.

(incidentally, out of curiosity, any reason for the migrate vs upgrade naming conflict in the Rakefile? :P)
use u-url in notification message if available