Add custom href for links

Add custom href for links. To support POST-form pagination you should write JS function.

pagination = Pagination(page=page,
                                      per_page=per_page,
                                      total=total,
                                      href='javascript:change_page({0})')

JS function will change page and submit form. Example from flask template:

    <script type="text/javascript">
        function change_page ( page_num )
        {
          document.searchform.current_page.value = page_num ;
          document.searchform.submit() ;
        }
    </script>