Add callbacks: onBeforePageSwitch, onAfterPageSwitch, onPageJump

This adds three much-needed callbacks. They are used this way:

````javascript
onBeforePageSwitch: function(oldPageNr) {
    // do something
}
````

````javascript
onAfterPageSwitch: function(newPageNr) {
    // do something
}
````

````javascript
onPageJump: function(oldPageNr, newPageNr) {
    // do something
}
````