Added toArray factory function & isEmptyObject check function

This could then be used like this...

```javascript
pivotal.getStories = function (projectId, filters, cb) {
    pivotal.apiCall("GET", ["projects", projectId, "stories"], filters, null, null, function (err, data) {
        if (err) {
            cb(err)
        } else {
            cb(null, pivotal.toArray(data, 'story'))
        }
    });
};
```