GetAll for multiple ids

Hello.
I'm not sure, i'm understanding it right. But I couldn't find a way to construct query to `GetAll()` method. For example, you have to find several keys from the user input. What is the proper way to do it?

I've done some simple fix, which lets to pass slices to GetAll, e.g.

```go
r.Table("users").GetAll([]interface{}{
    "348c4c7d-631e-41ce-bece-942d70385bd3", 
    "0a56526c-bd7b-4f87-bf05-d42b5a07e297"}).Run(session)
```