Limit data grabbing to what is available.

It was possible for the DataTable#_get_json_table method to attempt to
grab more data than available, resulting in an infinite loop. If user
provides a record maximum that is higher than the total records, only
grab the total records.
Adds method for setting table row size.

AJAX#_row_pref_of limits the row size to one of the legal values
supported by paginated controllers in Nexpose.
Adds past_scans method.

Method retrieves completed scan history for all sites on a console.