Paginate all, individual and company contributors

:fork_and_knife: The contributors lists are getting a bit long and costly when rendering the page so let's paginate them.
@brettchalupa I left a comment above. It wasn't clear to me what this expectation was testing. If it's testing what you say it's supposed to be testing it looks like this is already covered [here](https://github.com/opscode/supermarket/blob/paginate-contributors/spec/models/user_spec.rb#L123).