Fix #11366 Customised graduated renderer labels ignored

@NathanW2

This fixes bug http://hub.qgis.org/issues/11366 - customised labels in graduated renderer not respected.

Also includes code changes from pull request #1613 - I'll merge these back in to #1613 after this is applied. These code changes fix two small issues:
 * Missing PyName in SIP
 * Handling of negative zero values (avoids -0.00)