Adding delete collection

[JackLewis-digirati]

Resolves #22

This PR adds the ability to delete a collection

The format for this request is DELETE {base URL}/{customer id}/collections/{collection id}

In order to perform a delete, the request must be authorized and have a X-IIIF-CS-Show-Extras header

[donaldgray]

LGTM, couple of minor comments. Also, I'm not sure but should ETag be required for deletion?