As noted in #24, reading from the kqueue won't unblock when
it is closed on all platforms. This commit solves the issue by
additionally watching on a pipe which is closed when Watcher
is closed. The read timeout is no longer necessary.

The timeout for unix.Kevent() is causing issues; every 100ms it will do a new
unix.Kevent() syscall, which isn't too efficient: even if you have just one
change an hour, you will still keep calling kevent() ten times per second,
resulting in a needlessly high CPU usage.

Without a timeout, kevent() will block indefinitely until there are some events,
which is much more efficient. We can't just remove the timout however, since we
can't interrupt the kevent() call on FreeBSD and NetBSD, and it will hang
forever. This issue is described in more detail here:
#262 (comment)

To solve this we register a new kevent() with the file descriptor set to the
closepipe; when this pipe is closed an event is sent and kevent() will stop
blocking.

This is a rebased version of #124.

Fixes #89
Fixes #237
Fixes #333
Supersedes and closes #124
Supersedes and closes #262
Supersedes and closes #334