Support for metadata.json fallback for non-existent metadata.rb

Cookbooks do not neccessarily have metadata.rb files, although they are encouraged to and should, this is not enforced at least by the opscode community site. The majority do and use that to generate metadata.json, but it's the json file itself that chef uses, and hence there are some in the wild that do not contain a metadata.rb file (example: http://community.opscode.com/cookbooks/aide as of 0.1.1). Running into such an issue causes the metadata load to fail and berkshelf to not be able to support installing cookbooks with *only* a metada.json

This isn't likely to affect many cookbooks, but when it does, the only other options are to fork the cookbook, or vendor the cookbook. This is probably a 'nice to have' class feature.

note: Moving from (https://github.com/RiotGames/berkshelf/issues/418)