Add webmention support

![](http://dissociatedpress.com/wp-content/uploads/2011/02/oh-the-huge-manatee.jpg)
Looks like it's an ActiveSupport thing (according to [this blog post](http://blog.firsthand.ca/2010/07/ruby-objecttry-method.html)) and as luck would have it, I'm including ActiveSupport already for some date inflection.

Jeez this thing really should've just been a Rails app, yeah?