. support for diffing date/times through date_diff() / DateTime::diff().
. added DateInterval class to represent the difference between two date/times.
. support for parsing ISO intervals for use with DateInterval.
. date_add() / DateTime::add(), date_sub() / DateTime::sub() for applying an
interval to an existing date/time.
- Made the test cases work with the normal non-class constants.
- Merged new timelib, which is a bit more cleverer
- Support "UTC" in strtotime() properly.
- Added astro code, which is going to form the base for the new sunfuncs.
