improve STDOFF and DST offsets to handle one minute resolution

[bxparks]

Currently (v0.7.2), the AT and UNTIL times are handled with one-minute resolution. But the STDOFF and DST offsets (in RULES column with 'hh:mm' and SAVE column) are handled using a 15-minute resolution. The 15-minute resolution is sufficient for all timezones after the year 2000, but if we want to support historical DST shifts before year 2000, then we need to support one-minute resolution.

An example is Pacific/Kiritimati which used a UTC offset of -10:40 from 1901 to 1979:

# Zone  NAME            STDOFF  RULES   FORMAT  [UNTIL]
Zone Pacific/Kiritimati -10:29:20 -     LMT     1901
                        -10:40  -       -1040   1979 Oct                        
                        -10:00  -       -10 

If we go very far back, there are a number of timezones which have UTC offsets in one-second resolution. Because I want to keep the zoneinfo files as small as possible, I don't think these will be supported. For curiosity, here is the output from tzcompiler.py if we ask it to truncate the offsets to 1-minute resolution, for the years 1935 - 2050:

// Africa/El_Aaiun (STDOFF '-0:52:48' truncated to '-00:52')
// Africa/Johannesburg (RULES not fixed but FORMAT is missing '%' or '/')
// Africa/Monrovia (STDOFF '-0:44:30' truncated to '-00:44')
// America/Goose_Bay (STDOFF '-3:30:52' truncated to '-03:30')
// America/Managua (STDOFF '-5:45:12' truncated to '-05:45')
// America/Paramaribo (STDOFF '-3:40:36' truncated to '-03:40', STDOFF '-3:40:52' truncated to '-03:40')
// America/St_Johns (STDOFF '-3:30:52' truncated to '-03:30')
// Asia/Dhaka (STDOFF '5:53:20' truncated to '05:53')
// Asia/Riyadh (STDOFF '3:06:52' truncated to '03:06')
// Asia/Tehran (STDOFF '3:25:44' truncated to '03:25')
// Asia/Thimphu (STDOFF '5:58:36' truncated to '05:58')
// Atlantic/Azores (RULES not fixed but FORMAT is missing '%' or '/')
// Atlantic/Madeira (RULES not fixed but FORMAT is missing '%' or '/')          
// Europe/Amsterdam (STDOFF '0:19:32' truncated to '00:19')