add .fixture()

Kinda awkward for now i think, here's how you use it:

```js
it('should map basic identify', function(){
  test(keen, __dirname).fixture('basic');
});
```

but you can also:

```js
beforeEach(function(){
  keen = new KeenIO;
  test = Tester(keen, __dirname);
});

it('should map basic identify', function(){
  test.fixture('identify-basic', { someSetting: true });
});
```

cc @gjohnson @calvinfo