Handle cancelled selection keys... "properly"?

This seems very needlessly complex for something which shouldn't even
have to be an issue, but I can't obviously see how else to handle it,
but also not really what the whole point of selectors "cancelled-sets"
is.

Fix potential invalid select timeouts.

Log errors when closing watchers.

Added Watcher.added().

Actually implement event-driver timeouts.

Fixed heap removal bug.

Use -1 from Watcher.events() to signal closure, rather than 0.

Added a basic event-loop driver.

Initial commit.