Adds support for a global reporter function

This adds a simple globals hook for your own reporting code or module. I saw some grander discussion in #54, but this seems like a low-cost way of getting the feature started. 

My use case is just to print a summary of which tests failed (and how) so I don't have to spend a bunch of time combing through a hundred pages of scrollback looking for red lines after an hour-long test run.

Add a `reporter` key to your globals module with a function that accepts a test results object (like the built-in JUnit reporter does). The function is called after all tests finish. It may be asynchronous, with a 'done' callback as the second argument.

Reporter modules could be loaded via `require` and assigned to the globals `reporter` key if you wanted to get fancy. The feature could be expanded to allow specifying a path to a reporter module, too, if you prefer a JSON globals file.

I recommend starting with a simple `console.log` (as seen below) to view the test results data structure.

```javascript
// globalsModule.js
var util = require('util');

module.exports = {
  reporter: function(results) {
    console.log(util.inspect(results));
  }
};

/*
TEST FAILURE: 1 assertions failed, 0 passed and 0 skipped.. (9491 ms)
{
 "passed": 0,
 "failed": 1,
 "errors": 0,
 "skipped": 0,
 "tests": 1,
 "errmessages": [],
 "modules": {
  "tests/MyAppTest": {
   "My App Behavior": {
    "passed": 0,
    "failed": 1,
    "errors": 0,
    "skipped": 0,
    "tests": [
     {
      "message": "Timed out while waiting for element <//div[@testid='textLabel32' and contains(.,'AVERAGE')]> to be present for 5000 milliseconds.",
      "stacktrace": "Assertion failed in: Timed out while waiting for element <//div[@testid='textLabel32' and contains(.,'AVERAGE')]> to be present for 5000 milliseconds.: Expected \"visible\" but got: \"not found\"\n    at F.WaitForElement.fail (/home/mildmojo/projects/my_app/node_modules/nightwatch/lib/selenium/commands/_waitForElement.js:192:15)",
      "failure": "Expected \"visible\" but got: \"not found\"",
      "screenshots": [
        "/home/mildmojo/projects/my_app/tests/nightwatch/screenshots/ERROR_Sat-Oct-11-2014-185837-GMT-0400.png"
      ]
     }
    ]
   }
  }
 }
}
*/
```