gopath separator on darwin (OSX)

This will add support for OSX. It's exactly the same as linux - so some factorisation could be done.

Before:
```
$ go test
# github.com/andrebq/gas
./fs.go:64: undefined: gopathDirs
FAIL	github.com/andrebq/gas [build failed]
Exit 2
```

After:
```
$ go test
PASS
ok  	github.com/andrebq/gas	0.030s
```