Add page.exit()

Hello, After I went over all issues of page.js, I found out there are three issues(#38, #140 and #136 ) have same request of a exit function.

In my SPA scenario, I need exit function to dispose timer, dereference DOM elements, hide current div etc.

So here is my solution, please take a look my PR. People can use like this.

```
page('/app/home', main.loadData).exit(function(){
    //exit home page, some clean up work here 
});
```