Added a paging callback event

[raphaelstolt]

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 with the uglify_js compiler and optimization_level SIMPLE_OPTIMIZATIONS.

[raphaelstolt]

What's the status with this one?