Use built in is leap year function

Modified code to use the isleap() function in the calendar module.
Modified datetime/date_str functions

Modified datetime_to_date function to use built-in datetime strftime
method and modified date_to_datetime to parse the date string with a
regular expression.
Merge pull request #1 from cgmorton/devel

Basic modifications to date_util and form_util
Simplified set_back_date function using strftime method
Merge pull request #1 from cgmorton/master

Modifications to date_util.py and form_util.py to use built-in Python functions and methods