Fetch multiple batches of items from DPLA API

I added paging support so that multiple requests can be made to the API for more results (page size is capped at 100 results).  It will attempt to get up to 500 results currently (the value in the original code).

The results are supposed to be returned in order of increasing distance from the map center, but I don't think the API is doing this properly right now.  If they fix this, I'd be tempted to compute the map zoom from the results rather than the other way around ie fetch 100 results, then fit the map to their distribution.