Move Cookbook maintainer, description to CookbookVersion

Since maintainer and description can be changed overtime with new
versions there is no need to store these attributes on Cookbook. This
moves both of these attributes from Cookbook to CookbookVersion but
delegates Cookbook#maintainer and Cookbook#description to
Cookbook#latest_cookbook_version for easy access.
Merge pull request #337 from opscode/move-cookbook-attrs-to-cookbook-version

Move Cookbook maintainer, description to CookbookVersion