Initial specs and slight refactoring for Minitel

@brandur @pedro what do you think?
I mix this helper into tests that use Excon stubs:

```ruby
module ExconHelper
  def setup
    super
    Excon.stubs.clear
    Excon.defaults[:mock] = true
  end

  def teardown
    # FIXME This is a bit ugly, but Excon doesn't provide a builtin way to
    # ensure that a request was invoked, so we have to do it ourselves.
    # Without this, and the Excon.stubs.pop calls in the tests that use this
    # helper, tests will pass if request logic is completely removed from
    # application code. -jkakar
    assert(Excon.stubs.empty?, 'Expected HTTP requests were not made.')
    super
  end
end
```