Add functionality to build and install chef gem from client source code.

If your `.kitchen.yml` includes

```ruby
provisioner:
  name: chef_solo
  chef_git_sha: 9ec6b92b9eefc07b35e6d36bad3568cfd7bf82d4
  chef_git_url: "github.com/opscode/chef"
```

then `kitchen` will download the source code from the commit with matching SHA, build a gem from the source code using `rake`, and install the gem locally into `/opt/chef/`. Requires `chef_omnibus` or a previous installation of `chef`.