Re restore

Based on #10. Can now restore a piece of content. The revision fields and last modified timestamps are updated automatically.

A caveat is that you may find yourself having to modify the required fields in Composer to be able to publish the content again.

The API URL is still hardcoded. Let's have a chat about how to solve that in a way we're happy with.
It's not really security by obscurity; that would be relying entirely on dubious secrets to secure the app. There are many more layers of security involved (panda, SSH keys, etc). It's more about [defence in depth](http://en.wikipedia.org/wiki/Defense_in_depth_%28computing%29), although the discussion here is whether hiding such URLs are part of the security process or not. It could be argue it's not worth it, I don't know what the wider department thinks or does.

/cc @tackley @philwills ?

We've avoided exposing such URLs in public repos until now, but this commit is now public so unless you obliterate it from the history (and trust Github to purge it), it's now out there.