Add URL shortening views

Links to graphite images are completely unpastable to jabber/irc/mail as
clients mangle the humongous urls. These commits add a url shortener
that can shorten any graphite url from /what/ever/.... to
/S/something_short.

It can be used manually by prepending /s to any url. This will return a
shortened version of that url. Subsequent commits will integrate this
with the composer and dashboard, and maybe other places.
Integrate the URL shortener

- The composer gets a button to shorten the URL to the current graph.
- The menu that appears when clicking on a graph in the dashboard
  gets a similar item.
Merge pull request #158 from seveas/url_shortener

Make graphite's /render/?.... urls copy-pastable