Freshen codebase to use Supermarket site.
Support `:publish_to_supermarket` option in Releaser.

Note that the former option (`:publish_to_community`) will continue to
work for previously set up Rakefiles and Thorfiles.
Update README with Supermarket references.
Decrease ABC complexity in Releaser#initialize.
Merge pull request #6 from fnichol/supermarket

Add Supermarket friendliness