Replace stub_factory with stub_principals in Accounts.iterkeys()/search()

This is the code where I've encountered the problem:

>>> from kotti.security import get_principals, Principal
>>> p = Principal('p')
>>> DBSession.add(p)
>>> principals = get_principals()
>>> print principals.keys()
Merge pull request #1 from tiberiuichim/master

Replace stub_factory with stub_principals in Accounts.iterkeys()/search(...