Make munge integration tests more end-to-end

Ensure that the munge integration tests go through as much of the
pipeline as possible, including through @psd's fetching process. The
only part of the pipeline we don't touch is the curling of the correct
files down to the cache folder.
Extract cat process to seperate tool
Extract the whitehall mappings extraction into script
fix usage printout
Unit-style tests for csvcat
Added unit-style test for extracting mappings
Merge pull request #14 from alphagov/improve-munge-integration-testing

Make munge integration tests more end-to-end