Fixes Readdir when reading appended boxes.

Thanks for developing this package!

I was having trouble using Readdir against appended / zipped boxes. The Fileinfos I was getting back were nil. Example:
```go
    templateBox, err := rice.FindBox("tmpl")
    dir, err := templateBox.Open("/")
    finfos, err := dir.Readdir(0)
 
    // Load all files
    for _, f := range finfos {
        // f was returning nil...
    }
```

It also looks like there were problems with reading subdirectories. Please review my fix if you have a chance. Thanks!