Update Panopticon tests to minitest format

This renames test/panopticon_api_test.rb to become panopticon_test.rb, 
as per the naming conventions of other tests in this directory.

This also tidies up the tests to use the new Minitest-style test
definition methods, which will make it easier to add additional tests 
in the future.
Add methods to create, update and publish tags

This changes the Panopticon adapter to allow requests to be made to
create, update and publish tags.
Explicitly require panopticon test helpers

Not sure why this wasn't breaking before, but without this line the 
tests would blow up on a missing constant error.
Add new Panopticon test helpers (with tests!)

This adds three new test helpers to stub out the requests that clients
will make to Panopticon to create and update tags.

I've added unit tests for the test helpers here, which is something that 
hasn't previously been done before in this library. The tests assert 
that the correct stubs have been registered with WebMock for each test
helper.
Merge pull request #226 from alphagov/panopticon-tag-adapters

Add methods to create, update and publish tags in Panopticon