no id page back

if first page has no id, it can also go back.
callback和fallback上下文处理

callback和fallback上下文处理, 之前存在上下文可能会变成window的问题
Merge pull request #32 from zhangxinxu/develop

Develop
Merge remote-tracking branch 'upstream/master'
和张鑫旭版本同步