Added IntlTimeZone::fromDateTimeZone() and ::toDateTimeZone.

Added IntlTimeZone::fromDateTimeZone() and ::toDateTimeZone.

IntlTimeZone::fromDateTimeZone(DateTimeZone $dtz) converts from an
ext/date TimeZone to an IntlTimeZone. The conversion is done by feeding
the time zone name (essentially what would be given by
DateTimeZone::getName()) to ICU's TimeZone::createTimeZone except if it's
an offset time zone. In that case, the offset is read from the ext/date
time zone object structure and an appopriate id (of the form
GMT<+|-><HH:MM>) is given to ICU's TimeZone::createTimeZone. Not all
ext/date time zones are recognized for ICU. For instance, WEST is not.
Note that these kind of abbreviations, as far as I can tell, can only be
created via ext/date DateTime, not directly through DateTimeZone's
constructor.

For IntlTimeZone::toDateTimeZone(), the behavior is symmetrical.
We instantiate a DateTimeZone and then call its constructor if we don't
have an offset time zone, otherwise we mess with its structure. If the
timezone is not valid for ext/date, then we allow the exception of
DateTimeZone constructor to propagate.