Stop using PHPUnit Story

[Behaviour-driven development](http://www.phpunit.de/manual/3.6/en/behaviour-driven-development.html) is definitely a nice idea. However, the system currently used in wp-cli - namely PHPUnit Story - has several drawbacks:

### Hard to debug

If a scenario fails, the story runner doesn't tell you exactly which of the `then()` steps failed. If you try using the default phpunit runner, you also get a line number in `abstract-spec.php`, rather than in the file containing the scenario.

### Annoying syntax

The DSL provided by PHPUnit Story is rather annoying, making you constantly shift semicolons around. An interesting alternative would be http://behat.org/

### Awkward to use previous results

This is actually what prompted this decision: If I run a command that creates a user and returns the user id, how do I access that user id later, in the same scenario?