implemented order export, fixed issue #21 along with this

How to read the changes in ecommerce_order model:

Moved code, which prepares WHERE part of the SQL query to separate method prepareFilterWhereQuery($filter). This allowed me to reuse this code in the following new methods:

getOrderListCount() - get number of rows for pagination
getOrderListForExport() - new method which allows exporting of the filtered data. Has special flag $includeProducts, which will force export to include all purchased products along with order details.