Stateful set scaling

[doriordan]

This PR addresses several issues and enhancements related to StatefulSet and Scaling:

• fixes typo in its resource definition which resulted in incorrect API paths being used for this type for some API methods
• created separate version of the StatefulSet class for legacy v1beta1 version of the resource type under skuber.apps.v1beta1. This version of the class should be used with k8s v1.7 and earlier.
• updated skuber,apps.StatefulSet class to support k8s v1.8, including adding support for scaling stateful sets via the scale subresource.
• refactored scaling API making it possible now to add support for scaling of new resource types without modifying the APi itself. Older methods still supported for now but deprecated.
• updated ScaleExamples example to demonstrate scaling using new API and include stateful set scaling