check that the `tz` variable is set in setTimezone()
check that the `tz` variable is set in setTimezone()

Attempting to fix #16.

TooTallNate/node-time,santigimeno/node-time,TooTallNate/node-time,santigimeno/node-time,TooTallNate/node-time,santigimeno/node-time