Derive copy

An example of deriving a copy of something in archive:

POST /contents
`{"derivedFrom": "91cb5f28-2b8a-4324-9373-dac1d617bc24@1"}`

returns:
```
{"license": 
  {"url": "http://creativecommons.org/licenses/by/4.0/", "version": "4.0", "name": "Attribution", "abbr": "by"},
"language": "da",
"created": "2014-03-23T18:13:46.643205+01:00",
"derivedFrom": "91cb5f28-2b8a-4324-9373-dac1d617bc24@1",
"abstract": null,
"title": "Copy of Indk\u00f8b",
"modified": "2014-03-23T18:13:46.643374+01:00",
"mediaType": "application/vnd.org.cnx.module",
"content": "<html xmlns=\"http://www.w3.org/1999/xhtml\">\n...</html>",
"version": "draft",
"submitter": "cw0418",
"id": "1357ffcf-067b-4117-8a24-b0c8741c8e4e"}
```

This currently only works for modules.  I'll be working on collection next.