Define extra artefact factories using traits

This defines some additional factories that I wanted to use in the 
content api's tests, specifically around creating Whitehall artefacts 
and live/draft/archived artefacts.

For consistency, I've rolled the existing non_publisher_artefact factory 
into this too.
Merge pull request #121 from alphagov/building-moar-factories

Define extra artefact factories using traits