add Selection.All()

This was the only thing missing from your library that I really wanted.  I understand that using closures is the jQuery way, but it feels really awful in Go.  This method allows code like this:

```go
for i, s := range doc.Find(".reviews-wrap article .review-rhs").All() {
        band := s.Find("h3").Text()
        title := s.Find("i").Text()
        fmt.Printf("Review %d: %s - %s\n", i, band, title)
}
```

Presently, I *think* this might work, but it's pretty ugly:

```go
for i, n := doc.Find(".reviews-wrap article .reviews-rhs").Nodes {
        s := doc.End().AddNodes(n)
        band := s.Find("h3").Text()
        ...
}
```