Update screenshot.py

Changed how the difference between two screenshots is determined. It uses root mean square (code copied from: http://code.activestate.com/recipes/577630-comparing-two-images/) to determine how much it has changed. This overcomes the problem of a blinking network activity icon or blinking cursor from generating multiple screenshots.
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
Merge pull request #78 from carbito/patch-2

Changed how the difference between screenshots is determined
Added comments
Added other buttons to click
Merge branch 'development' of github.com:cuckoobox/cuckoo into development
Year bump
Bumped version
Added task deletion API
Fixed typos
Better relations constraints
Removed not used import
Updated API to delete task
Removed duplicated assignment
Merge branch 'development' of https://github.com/cuckoobox/cuckoo into development
bring back the repeated calls detection
fix missing last call of analysis
remove hook randomization
without virtualfreeex