Backwards incompatible changes

[Jef Driesen]

Hi,

I made two small backwards incompatible changes to the public api:

The dc_datetime_t structure has been extended with a new "timezone" 
field, containing the UTC offset in seconds. Devices without timezone 
support will set the field to the special value DC_TIMEZONE_NONE.

The hw_{frog,ostc,ostc3}_device_clock() functions have been removed in 
favor of the new dc_device_timesync() function.

Jef