Fix(recordsTotal): do not use search string for total count

[ghybs]

Hi,

This PR fixes https://github.com/jpravetz/node-datatable/issues/21

It adds a new parameter to buildCountStatement and buildWherePartial private functions, so that we can differentiate recordsTotal and recordsFiltered built queries.

See it in action: https://plnkr.co/edit/4VHcTgRzFziLQ9K8RUST?p=preview

[bean5]

@jpravetz This looks good to me. It just needs another test run.