0.7 - support DST transitions at 00:01; validate against Hinnant date library; upgrade to TZ 2019b

• 0.7
  • Change TimeZoneData to store mStdOffset and mDstOffset in units of
    one minute (instead of 15-minute increments, "code") in the off chance
    that the library supports timezones with one-minute shifts in the future.
  • Implement TimeOffset using 2 bytes (int16_t) instead of one byte
    (int8_t) to give it a resolution of one minute instead of 15 minutes.
  • Generate zoneinfo files containing AT and UNTIL timestamps with
    one-minute resolution (instead of 15-minute resolution). ZoneInfo files
    (zonedb/ and zonedbx/) remain identical in size. Flash memory
    consumption usually increases by 130 to 500 bytes, but sometimes decreases
    by 50-100 bytes. Timezones whose DST transitions occur at 00:01
    (America/Goose_Bay, America/Moncton, America/St_Johns, Asia/Gaza,
    Asia/Hebron) no longer truncate to 00:00.
  • Rename TimeOffset::forHour() to forHours() for consistency with
    forMinutes().
  • Make ExtendedZoneProcessor more memory efficient for 32-bit processors
    by packing internal fields to 4-byte boundaries.
  • Integrate C++11/14/17
    Hinnant Date library by
    creating additional tests/validation tests.
  • Upgrade zonedb and zonedbx zoneinfo files to version 2019b,
    after validating against the Hinnant date library.
  • Upgrade to pytz version 2019.2 to pickup TZ Database 2019b.