A YAM-like mainloop for Scala.

I seem to rewrite this code with every new language I learn.  The idea is to
provide an easy to use wrapper around select, with a thread-safe way to
schedule timers (and have new timers interrupt a blocking select).  This is
particularly nice in a functional language like Scala, where the callbacks
that you pass in to wait for a socket to become ready can be first-class
functions.

Other examples of a mainloop like this one can be found in libsfs, the Bamboo
DHT, and YAM (Yet Another Mainloop), which is part of the UIA project from MIT.