added moment().zoneExists()

As a result of an error fired when passing an invalid timezone name to ```moment().tz()```

I added the committed code to test for existence of a tz before attempting to localize to it - thought you'd like to see what I did.

I'm sure there's a better way of doing this, but ```moment().zoneExists()``` wasn't the code I deserved, just the code I needed right now.

Signed-off-by: Mike <mike@aviationshake.com>