Make Celluloid.new link by default if possible.

I've added an experimental change that makes `new` link by default if called within an actor's context. `spawn` remains unlinked by default and the `new_link` method will still require an actor context to link to, unlike `new`.

I've also added some minimal spec coverage but we might want to clean that up if we can decide on a clearer style to write these specs with proper actor cleanup.