Add file helper and ability to compare JSON with file content

Now you may have all your JSON examples in separate files and just load it in tests:

```php
$this->toJson()->shouldBeJsonEqualFile('user.json');
```

To specify directory which contains json files, just set `json_directory` property in json_spec settings in your `phpspec.yml`

FileHelper also available in behat:

```
Then the JSON response at "author" should be file "user.json"
```

Path to directory which contains json files also may be defined in behat.yml:

```
default:
    extensions:
        JsonSpec\Behat\Extension:
            excluded_keys: [id, created_at, updated_at]
            json_directory: './spec/support/files'
```

For more information, please see phpspec specs and behat features.