Update documentation and examples (#496)

Update documentation and examples (#496)

Update some documentation and add various examples. I added the examples
as subcommands of ./cmd/fsnotify, so they're directly runnable, and it's
at least guaranteed they compile.

This adds some simpler test cases so it's easier to verify it actually
works as documented on all platforms, and it adds internal.Debug() for
all platforms, which is useful in dev.

Fixes 49
Fixes 74
Fixes 94
Fixes 122
Fixes 238
Fixes 372
Fixes 401