Success and failed properties in after and arround hooks

Hi.  This is an implementation of #129.  Previously an AST Scenario object was passed into around and after hooks.   The request changes this to pass an API Scenario object into the hooks instead.  The API Scenario object has isSuccessful and isFailed properties: 

``` javascript
module.exports = function () {
  this.After(function(scenario, callback) {
    if (scenario.isFailed()) {
      // Do something like save a screenshot
      callback();
    }
    else {
      callback();
    }
  });
}
```