Fix #408 customer CSV export bug

I try to this the issue with minimum effort. I think it might need some refactory. I traced it and noticed that the download controller mixin sets `model` attribute to `Balanced.Download` object. 

```
Balanced.DownloadControllerMixin = Ember.Mixin.create({
    needs: ['marketplace'],

    download_model: null, 
    show_download: false,

    openDownload: function () {
        var uri = this.getSearchUri();
        var download = Balanced.Download.create({
            uri: uri,
            email_address: null
        });
        this.set('model', download);  // <----- here it is
        this.set('show_download', true);
    },
```

I don't think it is a correct way to do, as it overwrites original model object of customer controller (and all other controllers), which might servers other purpose. When the `model` object is set by download controller mixin, some black magic happens behind the scene (I don't really like these magic behaviors, those Ember.Bind things, explicit is better than implicit :S). Eventually, the `transactions` attribute is updated in a wrong way, leads to updating view, that's why the transaction data disappears from the UI. 

I set it as `download_model` instead of `model`

```
this.set('download_model', download);
```

This should work, however, I feel the download modal should be implemented as a view object instead of a controller mixin as all other modal views did.