Timezone Offset

Add `timezoneOffset` option, defaults to browser timezone offset, and use it for every moment instances.
Use `m.tz(timezoneName)` here instead, and require `moment-timezone` module that correctly resolves the offset that was at the moment you are going to display.

Example: the `Europe/Moscow` timezone has recently (2014-10-26) changed from +0400 to +0300. So, at 2014-10-25 the offset was +0400, and after the switch at 2014-10-26 02:00 +0400 which became 2014-10-26 01:00 +0300, the offset has become +0300.