Add resource set data source and resource

This is the first part of implementing #53.

Todo

[x] ~make testacc passes~ all client package tests pass and the new tests added in this PR
[x] Fix issues with resource sets being left over after tests. This is related to resource sets being looked up by name when it isn't a unique key
[x] Find a way to consolidate the TestMain setup used by the acceptance tests and the client tests
[x] Remove unused xo.getAllObjects compatibility for resource sets
[x] Refactor Client.GetResourceSets and Client.GetResourceSet to use a common method
[x] Decide what to do about the resource set client api that looks up by name.
[x] Implement update support for the resource
[x] Final review

This change will only work after this xen-orchestra change is released