Fix for "next page request" issue #29

Added parameter ($field) to getRequestForNextPage and
getRequestForPreviousPage methods, with default value as null. This
parameter should indicate whether the user asks for specific data field
pagination (called methods with parameter) or general data pagination
(called methods without parameter). Also fixed the wrongly stdClass
treating as array issue, and changed paging attribute to "paging"
instead of the wrongly "pagination".