Fix parsing issue in strtotime()

PHP can't discern the difference between d/m/Y and m/d/Y when just the date
is submitted to strtotime(). Unfortunately, strptime() is not available
until PHP 5.1.0. This patch forces datepickers to change their values to
YYYY-MM-DD upon submission to disambiguate parsing issues.

Fixes #832
Merge pull request #833 from greezybacon/issue/datepicket-date-format

Fix parsing issue in strtotime()

Reviewed-By: Peter Rotich <peter@osticket.com>