Add price field to studio & logic to show it

Add cosmetic price field to studio & logic to show it

- Price field added to Advanced Settings
- Function that checks for price to display like this:
    - If display_price is set, set price var to that
    - If shopping_cart price is set, overwrite price var
    - if price var is set, return that, otherwise return "Free"
- Test added to check that the function outputs correct price

When both the course_price field and shopping_cart price are empty or set to 0:
![906a1370-5587-11e4-9a3d-8941d1301d9a](https://cloud.githubusercontent.com/assets/3364609/5381428/060b0078-8056-11e4-934d-54733d63c4df.png)

Course Price is displayed as Free:
![3b5bf266-5589-11e4-89a6-b43cedd8dd9a](https://cloud.githubusercontent.com/assets/3364609/5381455/369fb8be-8056-11e4-8a94-6e4faea7c997.png)


When either course_price or shopping_cart price is set to a value:
![7fd435c2-5587-11e4-8d6d-2b7806d61e50](https://cloud.githubusercontent.com/assets/3364609/5381467/5906944a-8056-11e4-9f07-8090f4910238.png)

That Course Price is displayed:
![5d354e7a-53cf-11e4-90e6-032c0b78427a](https://cloud.githubusercontent.com/assets/3364609/5381470/678827e0-8056-11e4-8771-dacf5171bb43.png)

As a comment from a fly on the wall, I would love to see our platform's default be open and free courses to the extent that I would expect someone to need to add a Price field themselves if they wanted to communicate a price, rather than have all open courses list "Free" explicitly. It is true that ultimately the price is Free, but the important metadata to communicate for open course about pages would be start dates and other information already in that sidebar, not necessarily the clarification/clarity that it is Free.