Update the Dashboard cookbook share content.

When a user does not own any cookbooks, display the commands for sharing a
cookbooks and add a link to the full docs. This commit removed the info about
the Supermarket Knife plugin since users should be able to just use
regular-ole-knife (ROK) since it follows redirects.
Merge pull request #563 from opscode/dashboard_share_update

Update the Dashboard cookbook share content.