Add IndependentDisk#destroy to delete a disk

Once #destroy has been called, the IndependentDisk object
is then in a state where it contains the id of a disk that is no
longer present in the remote VcloudDirector system.

This is exactly the same scenario as if a disk is deleted via other
means, such as via the UI.
Merge pull request #141 from mikepea/delete_independent_disks

Add IndependentDisk#destroy to delete a disk