Android检查是否已经连接到网络
http://blog.csdn.net/kerenigma/article/details/7455905
http://bingan.diandian.com/post/2011-05-09/6213458
http://www.wmgod.com/article/source-page-kw-5039.html
http://blog.sina.com.cn/s/blog_9c51ede40100z0pu.html
