Add supermarket_attributes to Metadata

Allow the metadata.rb in a cookbook to specify arbitrary attributes for
use from Supermarket. The usecase for this right now would be setting the issues
and source URLs for that Cookbook instead of specifying them in the
Supermarket UI.
Fix typo in Metadata documentation.
Use individual URL attributes in metadata

Instead of having an arbitrary Hash/Mash of supermarket_attributes, use two
explicit URL attributes at the top level of the metadata: the source_url and
issues_url.
Ensure metadata responds to method before sending, use public_send
Merge pull request #1914 from gofullstack/supermarket_attributes_metadata

Add supermarket_attributes to Metadata