Add methods to create, update and publish tags in Panopticon

https://www.pivotaltracker.com/story/show/79178100

This changes the Panopticon API adapter to support creating, updating and publishing tags in Panopticon. 

* `GdsApi::Panopticon` gains three new methods: `create_tag`, `update_tag` and `publish_tag`.
* `test/panopticon_api_test.rb` is renamed to `test/panopticon_test.rb`, and the tests reformatted to use the MiniTest syntax (as per other tests here)
* Three new methods are added to `GdsApi::TestHelpers::Panopticon` to assist client apps with stubbing out requests for creating, updating and publishing tags.
* Unit tests are added for the new test helpers, asserting that they create the correct WebMock stubs for the requests they are stubbing. This is a first attempt at writing tests for test helpers - it's something we haven't done before.