Set cookbook source & issues url from metadata

:fork_and_knife: 

With the inevitability of chef supporting source_url and issues_url in the
metadata.rb, Supermarket should set the attributes on cookbook from the latest
version that gets uploaded.

The reason this does not add a source and issue url to CookbookVersion is - if
the URL changes, we would always want the URL to be the most recent one incase
they change. :smile:

Related to: 
- https://github.com/opscode/chef/pull/1914
- https://github.com/opscode/supermarket/issues/724
- https://trello.com/c/TKCg00fR