migrate clients to a separate table to allow easier communication and datasharing with other tools

To facilitate greater adoption in the enterprise and interoperability with other tools, we should allow a cleaner interface with the user/client combination. The user table is already fairly easy to access, being a separate table (as the hosting_user_client table). However, the clients listing is all in the shared node table, which make interoperability pretty hard, while it could be very easy to interoperate with other Drupals simply by putting it in a separate table.

I know this is a major architectural change again to a key structure that already got beaten up pretty bad in 0.2, but I think it's a necessary change in 0.3.