Next and Previous buttons not stopping at first/last

[Mark Mikofski]

expected: At last page pressing next stays on last page; at first page pressing prev stays on first page, and both should shows a toast see #13

current: Pressing next or previous shows the Oops page with "JSONException: No Value for Page."

[Mark Mikofski]

fix #25, for some reason the null in the JSON object for previous and next page urls is now a string!

→ 8e99c3e3ea1e