Allowing strings to be pulled from nested JSONObjects

Updating the optString method to take period delimited keys in order to fetch strings from nested JSONObjects.

Ex:
JSONArrayAdapter posts = new JSONArrayAdapter(getActivity(), posts, R.layout.simple_list_item_1, new String[]{"data.title"}, new int[] {R.id.text1});
would pull the "title" from the "data" JSONObject.