Add price field to studio and logic to show/hide price

@stvstnfrd & @jbau 

Screenshots to give a better idea of functionality:

Set "Course Price" in studio (Advanced Setting):
![screen shot 2014-10-16 at 3 54 36 pm](https://cloud.githubusercontent.com/assets/3364609/4671919/7fd435c2-5587-11e4-8d6d-2b7806d61e50.png)


"Course Price" is displayed on About Page:
![screen shot 2014-10-14 at 11 22 04 am](https://cloud.githubusercontent.com/assets/3364609/4634345/5d354e7a-53cf-11e4-90e6-032c0b78427a.png)




By default the field is set to 0:
![screen shot 2014-10-16 at 3 54 14 pm](https://cloud.githubusercontent.com/assets/3364609/4671928/906a1370-5587-11e4-9a3d-8941d1301d9a.png)


Which displays like this:
![screen shot 2014-10-16 at 4 07 11 pm](https://cloud.githubusercontent.com/assets/3364609/4672076/3b5bf266-5589-11e4-89a6-b43cedd8dd9a.png)

I don't really like overloading "or" like this.  I'd prefer just to be explicit about the order of precedence, something like
```
if registration_price != 0:
    return "{}{}".format(currency_symbol, registration_price)
elif course.display_price != 0:
    return "{}{}".format(currency_symbol, registration_price)
else:
    return _("Free")
```