Allow injectable/inheritance of Linq query provider

I have one final request for relaxing incaptulation in NH. These changes will allow me to create a custom Query Provider for ActiveRecord which properly manages the session lifecycle when executig a linq query.

The applied patch makes a couple of methods in Linq.NhQueryProvider virtual or protected.

It also moves the access of the internal session from PrepareQuery to Execute. If you can't make this change, could you make SetParameters protected instead?

Finally, it adds a constructor to Linq.NhQueryable which will allow me to insert my custom query provider when creating a Query.

Thanks.