Refactor pagination tag.

完成分页标签的统一，现在可以这样写了：

首页：

                <@paginationTag method="index" page="${posts.number}" total="${posts.totalPages}" display="3">
                    <div class="pagination">
                        <ul class="clearfix">
                            <#if pagination.hasPrev>
                                <li class="pre pagbuttons">
                                    <a class="btn" role="navigation" href="${pagination.prevPageFullPath!}">上一页</a>
                                </li>
                            </#if>
                            <#if pagination.hasNext>
                                <li class="next pagbuttons">
                                    <a class="btn" role="navigation" href="${pagination.nextPageFullPath!}">下一页</a>
                                </li>
                            </#if>
                        </ul>
                    </div>
                </@paginationTag>

标签归档页：

                <@paginationTag method="tagPosts" page="${posts.number}" total="${posts.totalPages}" display="3" slug="${tag.slug!}">
                    <div class="pagination">
                        <ul class="clearfix">
                            <#if pagination.hasPrev>
                                <li class="pre pagbuttons">
                                    <a class="btn" role="navigation" href="${pagination.prevPageFullPath!}">上一页</a>
                                </li>
                            </#if>
                            <#if pagination.hasNext>
                                <li class="next pagbuttons">
                                    <a class="btn" role="navigation" href="${pagination.nextPageFullPath!}">下一页</a>
                                </li>
                            </#if>
                        </ul>
                    </div>
                </@paginationTag>

分类归档页：

                <@paginationTag method="categoryPosts" page="${posts.number}" total="${posts.totalPages}" display="3" slug="${category.slug!}">
                    <div class="pagination">
                        <ul class="clearfix">
                            <#if pagination.hasPrev>
                                <li class="pre pagbuttons">
                                    <a class="btn" role="navigation" href="${pagination.prevPageFullPath!}">上一页</a>
                                </li>
                            </#if>
                            <#if pagination.hasNext>
                                <li class="next pagbuttons">
                                    <a class="btn" role="navigation" href="${pagination.nextPageFullPath!}">下一页</a>
                                </li>
                            </#if>
                        </ul>
                    </div>
                </@paginationTag>

另外，修改了一个变量。prePost -> prevPost