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 %>
Modify Link Rel helper method and Specs

1. Correct the example given in the comment to reflect the fact that absolute path is generated
2. Change the method name to be less verbose
3. Update method specs to make them more meaningful
4. Change logic in helper to be more DRY
Modify helper to be even more DRY

Finally saw what mislav was saying :P
Remove unneeded variable declarations
Modify helper method to use string interpolation and shorter conditionals.
Modify spacing in tags and add linebreak to example
Add support for params passed in to the controller

Since Kaminari supports params passed into the controller when rendering
link rels so that existing parameters (e.g. for searches) are maintained,
we should have that as well.
Merge pull request #1 from envato/feature/link_rel_support

Feature/link rel support