Switch to the new interface of exporter.

For OFX, the old interface is used as a proxy.
For QIF and GncXML, old interface and related methods in QifExpoter, GncXmlExport, Account (toQIF, toGncXML), Transaction (toQIF, toGncXML), and Split (toGncXML) are removed
Merge pull request #201 from fefe982/dev-exporter-interface

Changed Exporter interface for better memory efficiency