Added possibility to use a baseclass when auto-implementing an interface

A second $(REF AutoImplement, std, typecons) template has been added, which
differs from the existing one in accepting an extra type parameter. The extra
parameter specifies a class to derive from while auto-implementing an interface.

The base class used may contain non-default constructors. Matching constructors
will be created in the auto-implemented class and be implemented as just a call
to super with all arguments.
