ui.cli: Remove explicit return from QuestionCommandLine._extra_ps1_lines()

Mixing return statements and `yield`-based generators is bad practice.
README: Describe the types of supported questions
util: Support Python 3.2 by ignoring timeouts passed to invoke()

If you're not asking ScriptQuestions, you don't care about this
feature, and it's good to be able to relax the Python 3.3 requirement.