Query.first has unexpected behavior

```
func get(index: Int) -> Row? {
    let query = db["some_table"].limit(1, offset: index);
    return(query.first);
}

get(0) // gets row 0
get(3) // gets row 0
```

That behavior seems unexpected, it took me a while to figure it out. The problem is that `first` reruns the query with `limit(1)` and discards the offset: `var generator = limit(1).generate()`. Thanks for a great library.
