Updates to README

Update MongoEngine filters.py to include options description similar to BaseFilter

BaseFilter properly describes use of options. Including same wording for mongoengine filters.py

Docs

fix sorting with null optional value

Add Spanish translation

I didn't think to check the tests . . . sorry!

Return a 403 if a view class's is_accessible() returns false.

is_accessible says: Override this method to add permission checks.
This means that it should return forbidden since the item exists but you
are not allowed to see it

Fixed #318.

Datetimepicker