Fixed: getCurrentPage deprecated warning unclear

Refs #143
Docs: added inline callbacks deprecation notice

Refs #143
New: 'deprecated' message issued on first executeCallback() call

When the executeCallback function is called, we issue a message that tells the user that callback functions are deprecated. To issue the message only once, it was necessary to wrap executeCallback in a self-executing function in order to avoid declaring a firstRun boolean flag for the whole Diva instance.

Refs #143