ActionViewExtension refactored

This class has the [worst score on code climate](https://codeclimate.com/github/amatsuda/kaminari/Kaminari::ActionViewExtension), so I did some refactorings

 * duplication between link_to_prevous & next_page removed
 * Unnecessary complexity from rel_next_prev_link_tag removed
 * page_entries_info split into two easier methods