Add aggregate methods to IdentityFactory.

This allows things to create ConfigManagers without being exposed
to the underlying implementation.

Change-Id: I50e916909d83c6bd95dfbd1d61c446caade3ec02
Reviewed-on: http://gerrit.dmdirc.com/3079
Reviewed-by: Greg Holmes <greg@dmdirc.com>
Automatic-Compile: DMDirc Build Manager