Accept more date formats by using strtotime

Currently, the format for dates entered in the date range fields is hardcoded to dd-mm-yy.
This is rather unflexible, people like to enter dates in different formats.
Fortunately, PHP has a nice little function called strtotime that accepts lots of ways to enter a date and converts them into a timestamp.
The attached patch makes daterange.module use strtotime instead of the current code that only accepts one hardcoded date format.