Incorporate documentation command testing

[briandominick]

Use the Sourcerer module (currently in ReleaseHx but to be released as its own gem) to test commands in the docs.

In .adoc files, add [.testable] role to literalblocks that are meant to be run exactly as displayed, in an environment that follows from the sequence of commands before it, assuming bundle install up front.

Then add to Rakefile a task :readme_test like those in releasehx and releasehx-demo (neither yet updated as of time of this posting).