Simplify date conversion functions

Simplify date conversion functions

Do not use the 3rd-party code from JSC, but instead use the QDateTime
functions for millisecond offsets from the epoch.

Note that this change is reviving the earlier change:
http://codereview.qt-project.org/1874.  The test failures for that
change no longer occur due to a change in the QDateTime behavior
applied by: http://codereview.qt-project.org/13169

There is still a semantic difference between QDateTime and the
ECMAScript specification with regards to the application of DST.
ECMAScript requires that the current DST adjustment should be applied
to historical dates, whereas QDateTime will apply the adjustment as
applicable at the epoch date to any prior dates.

Change-Id: Ibebd47c9d2b75a034342ffcda075501a6a527b8d
Reviewed-by: Chris Adams <christopher.adams@nokia.com>