0.2 - better handling of gaps and overlaps in wall time with some date normalization

• 0.2 (2019-06-26, alpha)
  • Reduce flash memory size of WorldClock by removing extra font.
  • Split USER_GUIDE.md from README.md.
  • Rename ace_time::provider to ace_time::clock and rename
    SystemTimeProvider to SystemClock.
  • Add HelloSystemClock example code.
  • Add isValidYear() into various forComponents() methods to check
    int8_t range of year component.
  • Rename DateStrings::weekDay*() methods to dayOfWeek*() for
    consistency.
  • Change ZonedDateTime::printTo() format to match Java Time format.
  • Remove friend declarations not related to unit tests.
  • Remove redundant definitions of kInvalidEpochSeconds, standardize on
    LocalDate::kInvalidEpochSeconds.
  • Make timeOffset a required parameter for constructors and factory
    methods OffsetDateTime instead of defaulting to TimeOffset().
  • Make timeZone a required parameter in constructors and factory methods
    of ZonedDateTime.
  • Fix BasicZoneSpecifier::getOffsetDateTime() to handle gaps and overlaps
    in a reasonable way, and perform some amount of normalization.