Update API to allow summaries

[dosumis]

I've given up on the idea of bespoke filters & chose to provide the option of summary output instead.

Spec:
For get_instances & get_subclasses, remove the Filter option and add a optional boolean arg 'summary'.

This will require updating to the latest release. Please also note - one breaking change. get_instances on the API should be bound to vc.get_instances not vc.get_images

[dosumis]

BTW - performance with summaries set to True is v.slow. I'm planning to refactor to fix this. Probably over the weekend, but it would be good to have it live now anyway