Time decay boosting 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() and have not included a general boot of +1 as
this is only applied to documents where the public_timestamp exisits.

Also had to increase the boosts on the Inside Gov formats to compensate
for the time boosts.

This is for: https://www.pivotaltracker.com/story/show/42780733
Comments on the time_boost
Added tests for time decay
After working through this with david - decided on changing this into a
boost (not a penalty) as then the relavency score for older documents
will get lowered as the boot value is multiplied.

The result is that we can then revert the boost values on inside gov
formats

We tweaked the values of the curve so the maximum boost will be 2 (for
documents published right now). And generated a new graph in days so
it's actually readable.
Link to the graph not just the formual

Boost value vs days
Merge pull request #57 from alphagov/time-stamp-boosting

Time stamp boosting on documents with a field public_timestamp