Allow 'sort' option when requesting tags

This adds support for the 'sort' option when requesting tags
from the Content API.

It also includes a new test helper for stubbing requests for sorted
tags.
Allow 'sort' option when requesting child tags

This adds support for the 'sort' option when requesting child tags
from the Content API.

It also includes a new test helper for stubbing requests for sorted
child tags.
Merge pull request #169 from alphagov/content-api-tags-sort

Allow 'sort' option when requesting tags from Content API