convert from $0.99 to 0.99 USD (bug 1028361-two)

For in-app product page and payment pages
![screenshot 2014-08-27 09 14 04](https://cloud.githubusercontent.com/assets/74699/4063146/6de55110-2e05-11e4-9eba-8f33692bfb7e.png)

![screenshot 2014-08-27 09 13 37](https://cloud.githubusercontent.com/assets/74699/4063147/703ef272-2e05-11e4-98e6-d91a9f40de59.png)

Prevents awkward switch from $0.99 (which is ambiguous anyway, multiple countries use $ as their symbol) to 0.99 USD and displays it consistently in the UI.
Looks like you could do `Price.objects.active()`. Maybe it will be short enough to reformat at that point? Could leave it as-is but I don't like the break after the `=`.

```python
    price = forms.ChoiceField(choices=[(p.pk, p.tier_usd())
                                       for p in Price.objects.active()])
```