Graduated renderer unit tests2

@NathanW2 

This patch adds some python based unit tests to the QgsGraduatesSymbolRendererV2, and fixes one issue identified in the testing.

The issue is that the render state of each range is not restored correctly from the QDomElement - instead all the range render states were being reset to true.

The unit tests do not yet have complete coverage.  Mainly missing is calculation of ranges using the various methods, assignment of colours from the colour ramp, and calculating symbology for each feature (a fairly significant omission for testing a renderer!!)