Convert collections, to use persistent ids

Node.js util to convert urls to persistent ids, in local copies of collections. 
 
Output:
```
Mapped 432433394 <- technology/2014/mar/19/google-chromecast-uk-android-apps-digital-tv (3)
Mapped 432384233 <- money/2014/mar/18/childcare-plans-dismissed-au-pair-subsidy-labour (2)
Mapped 432378425 <- money/video/2014/mar/18/david-cameron-childcare-working-parents-video (1)
Mapped 432360908 <- news/reality-check/2014/mar/18/help-paying-childcare-tax-breaks-government (0)
Transformed: /home/stephan/gu/skeleton/util/map-ids/collection/uk-alpha/news/regular-stories/collection.json
```