Prevent recursion when handling "needy cards" on save.

"needy cards" mean cards other than the current card that have a bug
number but either need the bug description or need to create a bug
comment. needy cards occur when cards are saved while the computer has
no access to bugzilla.

	modified:   timecards/models.py
Added classmethods to generate timesheets

	modified:   timecards/models.py