Add basic fact cave lookup

Adds simple fact retrieval by slug, assumes a unique slug for now, this will change
when facts are keyed by date and slug.
Simplify method name
Fix test block spacing
Remove base_url method
Update var name to indicate single result
Stub full URL for fact
Handle an endpoint 404 gracefully
Return nil where the slug is nil or empty
Reword test descriptions
Add fact cave test helper
Add fact_cave_api helper
Refactor FactCave test helpers to use CommonResponses.
Remove unnecessary require
Merge pull request #66 from alphagov/add_fact_cave

Add fact cave client