Added a paging callback event

Allows to attach a callback to the left and right month and year navigation. 

`paging` callback parameter object:
```
{ unit="month|year", mm=7, yyyy=2015}
```

Is the documentation on http://freqdec.github.io/datePicker/ also available for editing via GitHub? Which JavaScript minifier is this project using? I just used [Sublime-Minifier](https://github.com/bistory/Sublime-Minifier) with the `uglify_js` compiler and optimization_level `SIMPLE_OPTIMIZATIONS`.