Non-generic Subscribe methods 

The ability to use non-generic methods for making subscriptions is useful when the subscription process is automated.

This feature has been requested a couple of times in the discussion group and it was easy to implement (besides the fact that I needed it myself), so here it is.

The code is fully backward-compatible, unit tests are passing (except for Housepipe which wouldn't run on my machine at all, but which shouldn't be impacted by the change).