Change default dates in database.py to callables

This does not affect submit.py, which creates a new Database() each
time it is run, but web.py and api.py both create a single Database
on startup, so previously all tasks submitted via these methods used
the date of startup rather than the actual submission date for Tasks.
Merge pull request #81 from MITRECND/date_fix

Change default dates in database.py to callables