Enable DAO.new to register unmigrated schemes

[dkent600]

The use case: One wants to create a new DAO and simulteneously register a scheme whose constructor requires an avatar address. Currently the only way to register such a scheme is to create the DAO and then propose to add the scheme via SchemeRegistrar.

So this ticket proposes that DAO.new support the provision of a callback that the caller will implement to instantiate a scheme once the avatar address is known. DAO.new will supply the avatar address as a parameter to the callback. The callback will return the new scheme's address and DAO.new will proceed to register it with the new avatar's controller.