behat: merge _run() and run() methods
behat: introduce Process class

The run_check() method is a shortcut for implicitly checking that a
command runs succesfully.
behat: set different db prefix for subdir install
behat: be more verbose when a command fails
generate test plugin on the fly, instead of downloading one from github
Merge pull request #434 from wp-cli/fix-434

Behat: "Given a google-sitemap-generator-cli plugin:" step broken