import the io package and returning nil so that go get doesn't return an error

It seems that d5a0140a6d3443678904ab30fc951193a25f8d4e broke the ability to `go get` this package. By importing "io" and returning nil, functionality doesn't change and `go get` works again. This snippet is from after I applied and pushed the attached patch:

```
[marpaia] ~ go get github.com/VividCortex/godaemon
# github.com/VividCortex/godaemon
go/src/github.com/VividCortex/godaemon/daemon_darwin.go:11: undefined: io
go/src/github.com/VividCortex/godaemon/daemon_darwin.go:12: missing return at end of function
[marpaia] ~ go get github.com/marpaia/godaemon
[marpaia] ~
```

Since I applied the patch on my own fork, the `go get` worked properly, etc.