Make serializers public

[aaronpowell]

I use a data migration tool to manage the setup of my MongoDB instances and seeding in the base data that is needed. Clients and Scopes are part of that base data but because this plugin uses a custom serializer for that it becomes problematic.

Is there any reason why the serializers couldn't be made public so they can be used without having to create the user service?

It'd also remove the need for InternalsVisibleTo for the tests (or at least, reduce the need for it)

[aaronpowell]

Actually, scratch that, it'll make things even worse for me as it embeds the v1.x MongoDB driver and I'm using the v2.x driver, so I'd have namespace/type collisions.

it'd be better if a custom serializer wasn't used at all...