added formatToken 'x' for unix offset in milliseconds #1938

I added the code for 'x' as a formatToken for both creating and formatting... I added the unittests. Hope it is sufficient like it is now...

I did have a problem with the name `unix offset` and it might be confusing with a timezone offset, but I am not a native english speaker so I just included `in milliseconds` everywhere I could. And the function `unixms` might not be needed, it's probably better to use valueOf(). But I leave that decision making up to you....