Updated the FeaturesContext for Behat 3.0-RC

This also switches to using the Process component as done in Behat.
Updated the extension class and the steps to use the Behat 3 interfaces

The feature of the extension is not implemented anymore for now as the
code changed entirely between Behat 3 beta and Behat 3 RC
Updated the scenarios for the new Behat output format
Added the output on failure
Implemented the SubStepTester
Updated the features for the new output of the progress formatter
Merge pull request #3 from stof/update_extension

Update the extension for Behat 3 RC