Allow the use of created and modified dates for API.

The API will now detect if you are using `c-created` or `c-modified` and
convert the string value to a datetime object. This will allow you to do
accurate querying with operators such as `__gt`, `__gte`, `__lt`,
`__lte`, etc.
Don't create new identifier on attribution.

Identifier has to exist on attribution. This prevents appending
organizations to source lists that didn't provide that identifier.
Merge branch 'master' into stable_4