SegmentIO constant is set in the Kernel namespace

`self` in `after_fork` is an instance of `Unicorn::Configurator`, so
we were previously setting a constant in that namespace. Then, in
controllers, we were unable to find the SegmentIO constant
Add docs badge to README
Merge pull request #228 from rrrene/patch-1

Add docs badge to README
Move README badges below the h1.

As the number of badges grows, it gets difficult to manage and wraps strangely. This moves it below the h1.
Merge pull request #227 from opscode/fix-segment-io-constant-missing

SegmentIO constant is set in the Kernel namespace
Adds a slug to Categories, to make searching and URLs easier
Merge pull request #229 from opscode/category-filtering-bug

Adds a slug to Categories, to make searching and URLs easier
Add titles and descriptions to applicable views.
Merge pull request #230 from opscode/titles

Add titles and descriptions to applicable views.
Cookbook source url and issues url can be managed.

- Add UI on cookbook show
- Rename 'external_url' to 'source_url' on Cookbook.
- Add 'issue_url' attribute to Cookbook.
Merge pull request #212 from opscode/manage_cookbook_urls

Users can manage cookbook url data
Update Virtus.
Add breadcrumbs to Cookbook show.
Merge pull request #232 from opscode/update_virtus

Update Virtus.
Merge pull request #233 from opscode/cookbook_show_breadcrumbs

Add breadcrumbs to Cookbook show.
Replace Devise authentication with ChefOAuth2.

[ci skip]

This is not yet complete, as oc-id is not deployed in production and the
omniauth strategy is not open source (Travis CI builds cannot pass).

This replaces Devise (email/password) authentication with Omniauth
authentication. Supermarket uses oc-id, a Chef service, as the provider for
authentication. This means that for a user to sign up and/or sign in to
Supermarket they must create a Chef Server account, as they currently do. This
however removes the duplication of records and simplifies the communication
between the community site and Chef Server for authentication when sharing
cookbooks.

Notes:

* Remove omniauth-twitter because it is no longer being used
* Store chef_oauth2 public key on Users.