Replace connectable signals with multicast connections

This replaces `RACConnectableSignal` with `RACMulticastConnection`, as discussed in #214.

I also removed `RACCancelableSignal` since it was a subclass of `RACConnectableSignal` and it seemed silly to spend time updating something that was going away. (#73)

There are a couple open questions still:

1. In #214 we talked about dropping `-autoconnect`, but it seems like we still need something like that which disposes of the shared subscription automatically. I made changes to `-autoconnect` to make it safe compared to what it was.

2. Should `-replayLazily` use `-autoconnect`? I'm leaning towards yes.