Add #rel_next_prev_link_tags helper method

This adds a #rel_next_prev_link_tags method to will_paginate. Essentially
taken straight from [Kaminari's implementation](https://github.com/amatsuda/kaminari/pull/200/files)

    #In head:
    <head>
      <title>My Website</title>
      <%= yield :head %>
    </head>

    #Somewhere in body:
    <% content_for :head do %>
      <%= rel_next_prev_link_tags @items %>
    <% end %>