Timezone Offset

Add `timezoneOffset` option, defaults to browser timezone offset, and use it for every moment instances.
Yeah, I think the idea was to keep the dependency of `moment-timezone` out of this module, and let the app that instantiate the calendar passing the current "today's" timezone offset (that can be acquired via moment timezone indeed).
I closed the PR anyway as I ran into issues with it, and decided to prepare my moment objects before passing them to the calendar.