Initial specs and slight refactoring for Minitel

@brandur @pedro what do you think?
Here's an example of a test.  It's ugly, but you have to call
`Excon.stubs.pop` in the stub block for things to work as expected:

```ruby
  # Link.run injects parameters into the path in the order they were received.
  def test_run_with_parameters_and_empty_response
    Excon.stub(method: :get) do |request|
      assert_equal('/resource/44724831-bf66-4bc2-865f-e2c4c2b14c78',
                   request[:path])
      Excon.stubs.pop
      {status: 200, body: ''}
    end

    schema = Heroics::Schema.new(SAMPLE_SCHEMA)
    link = Heroics::Link.new('https://example.com',
                             schema.resource('resource').link('info'))
    assert_equal(nil, link.run('44724831-bf66-4bc2-865f-e2c4c2b14c78'))
  end
```