Allow voting on reviews

First draft, posting a PR mainly since I have a few questions, and for some early feedback. 
![rate_reviews_1](https://cloud.githubusercontent.com/assets/2644780/3867707/5265ae9c-200b-11e4-83eb-ac71a6d84bc3.png)

You should leave the original implementation for `map_field()`.

Instead, change

```python
return {f: map_field(f) for f in fields}
```

to

```python
uc_dict = {f: map_field(f) for f in fields}
uc_dict.course_review['user_course_id'] = str(self.id)
uc_dict.professor_review['user_course_id'] = str(self.id)
```

so it's clearer what the 'user_course_id' is being added to.