Pull URL creation out to BuildURL func

We are going to need to manage at least 3 different URLs so it made
sense to pull this function out.
Throw an error when performance platform does

We were gobbling up all of the error information returned from a bad
request. We should surface this by emitting an error with the attached
message.
Refactor info_test to use fixtures for JSON
Refactor to proper client

Removed assertions from info_test as it is no longer called in main
Add SlugStatistics pulling page views
Add in number of searches to statistics
Add search terms to SlugStatistics
Merge pull request #1 from alphagov/feature/performanceplatform-api

Feature/performanceplatform api