导航栏背景图片和按钮式样

Squashed commit of the following:

commit edb90fa27c5ac44804352a71680aa92c774ed118
Author: 张岭 <zhangling@babytree-inc.com>
Date:   Sun Jan 20 19:40:09 2013 +0800

    导航按钮式样

commit 4620bf2d86b571671d40b5ec085a2b3a909604c0
Author: 张岭 <zhangling@babytree-inc.com>
Date:   Sun Jan 20 19:26:40 2013 +0800

    添加导航栏背景图片