Added callback to preprocess 

This adds the ability to hold up page rendering until content is fully prepared making for a smoother experience vs switching to a blank page with a loading icon.

example usage as follows

```
var $$=Framework7.$,
        myApp= new Framework7({       
        preprocess: function (content, url, callback) {
           var xhr, template;
           if(url == 'login.html') {
               myApp.hideIndicator();
               callback( content );
           }
                xhr = $$.ajax('someurl.html');
                template = Handlebars.compile(content);

            $$.when(xhr).then(function (data) {
                if (data.code === 200) {
                    myApp.hideIndicator();
                    callback( template(data) );
                }else if(data.code === 204){
                    myApp.hideIndicator();
                    myApp.allowPageChange = true;
                    mainView.loadPage('login.html');
                }
            });
        }
    });
```