Move JavaScript functions to the Datetime Object

[nilshoerrmann]

The current structure of the JavaScript cannot be read and understood easily.

[nitriques]

I've seen worst! and really worst! :smile:

[nilshoerrmann]

Some notes:

• This extension should make use of the Symphony.Extensions namespace.
• It should adopt the core calendar (if it stays in the core) or integrate it (if it's removed from the core again).

[nitriques]

:+1: