Allow specifying accounts by id in manage.py

[frohoff]

The manage.py/monkey script currently only supports specifying accounts by their security_monkey-managed name when running tasks such as find_changes. This PR adds support for specifying accounts by their id (acccount.identifier) and allows for easier integrations with external manual or automated workflows for running tasks that only are aware of the identifier.

$ python manage.py find_changes -a 1234567890 -m s3

[coveralls]

Coverage decreased (-0.02%) to 63.617% when pulling c59de32 on frohoff:manage_by_account_id into 21cfeb7 on Netflix:develop.