POST to Fieri after a cookbook upload.

:fork_and_knife: 

If Fieri is enabled and the FIERI_URL is configured, send a POST request to the
configured FIERI_URL after a cookbook version is successfully shared. This sends
along the cookbook name, the version and a link to the artifact in S3. Fieri
will then do what it needs to do and then send a POST request back to
Supermarket when it is done evaluating the cookbook.

This does not implement the end-point for Fieri to use, it just the first part
of making this all happen.

Trello card for this: https://trello.com/c/P6WemEzu

The blob that gets sent to Fieri:

``` json
{
  "cookbook_name": "redis",
  "cookbook_version": "1.2.0",
  "cookbook_artifact_url": "https://link-to-cookbook-archive-on-s3-or-cdn.com/redis-v1_2_0.tgz"
}
```
This _should_ theoretically be fine but if you wanted to be 100% sure that you're acting on the version that was just uploaded you could have [`Cookbook#publish_version!`](https://github.com/opscode/supermarket/blob/master/app/models/cookbook.rb#L192) return the version that was just published instead of `true`. Then in [`CookbookUpload#finish`](https://github.com/opscode/supermarket/blob/master/app/models/cookbook_upload.rb#L26) you could yield the cookbook (result) and the cookbook version returned by `Cookbook#publish_version!` you'd then be able to have access to the version in the finish block in the [controller](https://github.com/opscode/supermarket/blob/master/app/controllers/api/v1/cookbook_uploads_controller.rb#L51).