Add default interface

@shadyvb I've added a default interface to the WP_Stream_Connector class. I've also added the final keywords on functions I think shouldn't be overridden (sometimes we provide a filter so it's kind of useless that we allow it, but I'm open to discussion).

I've also added a condition to check if a third party connector doesn't extends the main connector class, we skip the registration.

This should solve #96