Rename playd class to Playd.

Fixes #39.
Rename IoReactor class to IoCore.

Because 'reactor' is a pretty weird term for people who don't know what
a reactor is in terms of asynchronous programming, and it isn't even
strictly correct if they do.
Rename TcpResponseSink class to Connection.

Removes a TODO.
Catch a remnant of IoReactor missed earlier.

@wlcx, thanks for pointing this out!
Merge pull request #42 from UniversityRadioYork/mw-lipstick

Three class renames.