Standalone app (aka slimmer as a service)

**DO NOT MERGE: This is a prototype for discussion**

Here's a very quick first pass on providing a rack application that makes slimmer's behaviour available over HTTP. It provides most of the functionality of slimmer but won't use the correct asset host, allow changing templates, or accept any arguments that are currently passed via HTTP headers other than the artefact header.

To get this running you'll need to have run ``bundle install`` in the main slimmer folder. Then change into app and run:

```bundle exec rackup config.ru```

That will start the app on port 9292. It expects a POST to http://localhost:9292/ with a JSON body that's a hash with keys that look like an Artefact plus a key of source_content containing the HTML you want to skin.