Refactor TaricImporter

Split it out into smaller components and make more object oriented -
testable.

Also ensures that conformance validations happen in reverse (to
transaction) sequence to avoid false positive conformance errors. E.g.:
footnote gets inserted before footnote description period and we get an
error about footnote missing at least one description period. This
changes the sequence so that description period gets inserted (and
validated) first.
Adjust namespace for attribute mutators

Also add integration spec for TaricImporter to make sure we import
complex entities like Measure
Merge pull request #94 from alphagov/taric-validator-fix

Taric importer refactoring and validation fix