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
Merge pull request #28 from snarfed/u-url

use mf2 u-url property when available. fixes #23