Time decay boosting/penalty for content with a public timestamp

Implemented using a custom score as in this blog post:
http://jontai.me/blog/2013/01/advanced-scoring-in-elasticsearch/

Changed `now` to `time()` (as now was not defined) and have not included a general boot of +1 as this only applies to documents where the `public_timestamp` exists and we want it to behave as a decay penalty.

I also had to increase the boosts on the Inside Gov formats to compensate for the boosts on documents with `public_timestamp`.

This is for: https://www.pivotaltracker.com/story/show/42780733