Length() function

It would be cool to have a shorthand way of getting the length of a JSON array. For example with this JSON:

``` json
{
  "data" : [1, 2, 3]
}
```

... I currently need to do this to check the length:

``` go
arr, _ := jq.Array("data")
fmt.Println( len(arr) )
```

It would be nice to be able to do this:

``` go
fmt.Println( jq.Length("data") )
```

A small thing, but very handy.
