Changes in QGis 2.0 API

Since QGis 2.0 API has been cleaned up, some methods don't exist anymore (for instance isUsingRendererV2)

Changes have been made to take those cleanups in consideration.

Commit proposed has been successfully tested on QGis 1.8 (old and new symbology) and QGis Master (new) (Ubuntu)

Hien TRAN-QUANG