DateFormat plays nice with Calendar, TimeZone

The following changes were made:

* The IntlDateFormatter constructor now accepts the usual values
for its $timezone argument. Thi

DateFormat plays nice with Calendar, TimeZone

The following changes were made:

* The IntlDateFormatter constructor now accepts the usual values
for its $timezone argument. This includes timezone identifiers,
IntlTimeZone objects, DateTimeZone objects and NULL. An empty
string is not accepted. An invalid time zone is no longer accepted
(it used to use UTC in this case).
* When NULL is passed to IntlDateFormatter, the time zone specified in
date.timezone is used instead of the ICU default.
* The IntlDateFormatter $calendar argument now accepts also an
IntlCalendar. In this case, IntlDateFormatter::getCalendar() will
return false.
* The time zone passed to the IntlDateFormatter is ignored if it is
NULL and if the calendar passed is an IntlCalendar object -- in this
case, the IntlCalendar time zone will be used instead. Otherwise,
the time zone specified in the $timezone argument is used instead.
* Added IntlDateFormatter::getCalendarObject(), which always returns
the IntlCalendar object that backs the DateFormat, even if a
constant was passed to the constructor, i.e., if an IntlCalendar
was not passed to the constructor.
* Added IntlDateFormatter::setTimeZone(). It accepts the usual values
for time zone arguments. If NULL is passed, the time zone of the
IntlDateFormatter WILL be overridden with the default time zone,
even if an IntlCalendar object was passed to the constructor.
* Added IntlDateFormatter::getTimeZone(), which returns the time zone
that's associated with the DateFormat.
* Depreacated IntlDateFormatter::setTimeZoneId() and made it an alias
for IntlDateFormatter::setTimeZone(), as the new ::setTimeZone()
also accepts plain identifiers, besides other types.
IntlDateFormatter::getTimeZoneId() is not deprecated however.
* IntlDateFormatter::setCalendar() with a constant passed should now
work correctly. This requires saving the requested locale to the
constructor.
* Centralized the hacks required to avoid compilation disasters on
Windows due to some headers being included inside and outside of
extern "C" blocks.

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Added private constructor to IntlTimeZone.

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

IntlTimeZone::fromDateTimeZone(DateTimeZone $dtz) converts from an
ext/date TimeZone to an IntlTimeZone. The conversion is do

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.

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Added support for new method TimeZone::getUnknown() added in ICU 49.

Initial checkin of calendar/timezone code.