LP#1552778: copy some date/time utils from OpenSRF

LP#1552778: copy some date/time utils from OpenSRF

As preparation for subsequent bugfixes, this patch
copies several date/time routines from OpenSRF::Utils
to a new module, OpenILS::Utils::DateTime. Specifically,
the routines copied over are:

* clean_ISO8601() (renaming of the OpenSRF cleanse_ISO8601)
* gmtime_ISO8601()
* interval_to_seconds()
* seconds_to_interval()

This move will allow us to fix bugs in this core routines
without requiring a mandatory OpenSRF upgrade. Furthermore,
with the exception of interval_to_seconds() (and in only one
place), none of those routines are used by OpenSRF itself.

To test
-------
[1] Apply the patch.
[2] Verify that unit tests pass.
[3] Verify that all Perl services start correctly.
[4] Verify that date/time and interval calculations continue
    to work as expected, particularly in circulation.

Signed-off-by: Galen Charlton <gmc@equinoxinitiative.org>
Signed-off-by: Kathy Lussier <klussier@masslnc.org>