This is an abstract base class for objects backed by a socket which
provides the basic framework for event-driven asynchronous IO,
designed to be fast.  Danga::Socket is both a base class for objects,
and an event loop.

Callers subclass Danga::Socket.  Danga::Socket's constructor registers
itself with the Danga::Socket event loop, and invokes callbacks on the
object for readability, writability, errors, and other conditions.