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:
![screen shot 2014-10-14 at 11 19 37 am](https://cloud.githubusercontent.com/assets/3364609/4634338/4edd0520-53cf-11e4-98ee-d754688567f5.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)

Even if this lookup were free, we shouldn't be invoking it
consecutively. We should memoize this.

```python
price = get_course_about_section(course, 'price')
if price:
    ${price}
else:
    ${_('Free')}
```