Return exit code 0 instead of 2 when examples are skipped

Skipping examples should not make the spec suite fail (which is the case today).

I've updated the behat step to actually check the application exit code. (Related issue: https://github.com/phpspec/phpspec/issues/315)
> I think the naming in the PHP documentation is a confusion with the HTTP status code

I don't really think so. I know [libc](http://www.gnu.org/software/libc/manual/html_node/Exit-Status.html), [bash](http://www.gnu.org/software/bash/manual/bashref.html#Exit-Status), etc. also refer to it as an `exit status`. That's why I mentioned that `exit status` is probably the most common terminology.

Anyway, I've also seen `return code`, `exit code`, etc. in a number of places. Probably not a huge deal either way.