Add a length method to ListObject.

ListObjects behave much like arrays, and so I'd expect the length method to exist. `count` is available via the API results, but it seems that always returns the full number of records, not the number of records in the current list. Yes, I could just use `data.length` instead, but better to take the path of least surprise.

I'd also recommend changing count to be the number of records returned, and change the existing attribute to something like total_count, as (at least in Ruby) count and length are synonyms.