Add confirm and instruct prompts, avoid use of futures

This patch implements a blocking response queue for certain messages sent to
the server.  These messages are confirmPrompt, prompt, and instructPrompt.

This makes tornado's yield-based futures redundant, simplifying the tests quite
a bit and makes it possible to follow the same patterns for failing tests in
the harness code as MinimalTestCase uses.
Don't invalidate self.app because it's needed in close_cert_app
Merge pull request #34 from andreastt/ato/prompts-2

Add confirm and instruct prompts, avoid use of futures