POST to Fieri after a cookbook upload.

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.
CookbookUpload#finish yields the cookbook version.

Adjust Cookbook#publish_version! to return the cookbook version that is
published. Use this to yield the cookbook_version within CookbookUpload#finish.
This is then used in the cookbook uploads controller spec for sending the
version number to Fieri.
Merge pull request #741 from opscode/supermarket_to_fieri

POST to Fieri after a cookbook upload.