Fix go get and darwin support.

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.

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Also, since `Daemonize` is only kept for backwards API compatibility and `godaemon.MakeDaemon(&godaemon.DaemonAttr{})` is the preferred way to create a daemon, `MakeDaemon` doesn't work on OS X because it was never added to `daemon_darwin.go`. I added it to `daemon_darwin.go` as a no-op, just like `MakeDaemon` so that at least code will compile.

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Also, since `DaemonAttr` is needed in both `daemon_linux` and `daemon_darwin`, I moved it to a new file, `daemon.go` so that it is accessible by both files.