Graduated renderer unit tests

Fixes:

* Missing PyName in SIP
* Handling of negative zero values (avoids -0.00)

Adds:

* Python unit tests for Graduated Renderer (incomplete).  So far covers QgsRendererRangeV2 and QgsRendererRangeV2LabelFormat.

Hope to find time to add (non-graphical) tests for the actual renderer - but creating pull request now in case that takes too long!