Add support for case independence when using toposort.

[KSchopmeyer]

the python package toposort is used to sort classes for at least the namespace delete function to dependency sort classes. However toposort is not a case-independent function and CIM classnames are. We need to extend this capability to allow case-independent toposort.

Moved from a TODO to this issue as of the release of pywbemtools 0.9.0