Ensure channels don't get stuck completely unregistered. (apple#104)

Motivation:

It should be possible to have channels that are not registered
for any form of I/O without them getting stuck in that model
forever.

Modifications:

Remove the code that prevents channels registered for `.none` from
registering for anything else.

Result:

Channels can actually be registered for nothing without becoming
wedged open forever.

fix readme dependencies (apple#42)