Rename `terminate` method to `close`?

[ryanfobel]

@cfobel We had discussed renaming the terminate method to close. I see that you've created an alias in this project, but should we do it in the base-node-rpc instead? I like this idea.

Also, I would like to expose a method to determine if a proxy is connected (e.g., for a serial proxy, this would just return proxy.serial_thread.alive). Do you prefer: proxy.connected, proxy.alive, proxy.open, something else?

[ryanfobel]

We could also use bind()/unbind() and bound; i.e., a proxy is "bound" to a process or processor.