export spaces defaulted to null

Related to [issue #15](https://github.com/jprichardson/node-jsonfile/issues/15).

`jsonfile` is basically a wrapper to `JSON` with additional functionality from `fs`. Thus, it should show the same behavior as `JSON`. When [`JSON.stringify()`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/JSON/stringify) is not supplied with an argument for `spaces`, it will not beautify the JSON output. Similarly, `jsonfile` should default to *not* beautifying the JSON output.