Reinstate dimensions in the _id field

Because they are a unique part of the record. Removing it in previous
commit 7cf43e52f17c709d2c4035537b2f2bbb16de5143 caused a regression:
https://www.pivotaltracker.com/story/show/63703882
Made ``_id`` field human-readable.

Because I think we would have spotted the missing data much more easily
had the id not been obfuscated.
Merge pull request #22 from alphagov/dimensions-in-id-field

Reinstate dimensions in the _id field