Add support for Sequel ORM

[kamui]

The sequel orm has it's own #pagination method. will_paginate doesn't override or wrap this method, it instead just uses the built in sequel pagination method. However, sequel's #paginate has a different method signature, instead of a hash or named parameters, it looks like this:

   def paginate(page_no, page_size, record_count=nil)
     # ...
   end

I couldn't think of a cleaner way to support Sequel, so what it does is check to see if collections inherits from Sequel::Dataset and if it does, it'll call #paginate with Sequel's method signature. If you have a better way to implement this, let me know and I'll do my best to modify this PR.

[kamui]

BTW will_paginate supports sequel, but mislav recommends when you call paginate, you use sequel's paginate method signature. So, that's why this bug exists when you use sequel and will_paginate in api-pagination.

[davidcelis]

Hey, this looks good to me. Thanks for submitting a fix! Would you mind squashing before I merge?

[kamui]

There you go.

[davidcelis]

Thanks! Looks like the build issues are related to travis-ci/travis-ci#2220