Reverting to a base64 encoded _id

Unfortunately human-readable _id was no good, as dimension with a space
(ie User Agent) would form an invalid id.
Add a humanId field to documents.

This is to aid debugging when looking at data in buckets. See related
bug https://www.pivotaltracker.com/story/show/63703882
Merge pull request #23 from alphagov/base64-encode-id

Base64 encode the _id