Fix local_date_time type processing

The local_date_time field type's raw_value is the offset in seconds
since 00:00 Dec 31 1989 LOCAL TIME. As the timezone offset is already
included in the value processing must use utcfromtimestamp.

Additionally the previous code was dependant on the local timezone of
the fitparse process.

If you don't like the POSIX_OFFSET constant, feel free to merge without this change. I just thought that it would be nice to see where this magic value comes from.