New decorator to retrieve test data from JSON files

Hi @txels

I've added a new decorator ```file_data``` that will load test data from a JSON file. 

At the moment the JSON file can only contain a flat list of values but if you like this idea/approach I can add support to a schema like the following

```javascript
[
 { 'test_name':  [1, 2, 5, 6],
   'test_name_2': [10, 33, 5]
 }
 ...
]
```

This would create 4 tests with the ```test_name``` appended to the original test function name (or even replaced but this might be confusing) and 3 more tests with ```test_name_2``` appended to the name.