Fix for https://github.com/spark/firmware/issues/280

This will fix https://github.com/spark/firmware/issues/280 for:

1.) Changed `Get_UnixTime` to factory in the **time_zone_cache** parameter

2.) Change the `Time.zone()` formula to multiple GMT_offset by 1800 instead 0f 3600 as tests showed that the original increase/decrease 2 hours per UTC

Sample code for test verification is here: https://gist.github.com/kennethlimcp/fe25df7bc1619654ff76