[DO NOT MERGE] 559 make media api more consumer friendly

I have added 'field_xpath' field on the media object which contains the question but am not sure if this was what was required
![screen shot 2014-11-06 at 3 02 18 pm](https://cloud.githubusercontent.com/assets/1128227/4935464/32caa0aa-65ad-11e4-9922-a43a32692759.png)

Wow, I didn't know there wasn't a simpler way to get the key for value in Python, but [apparently not](https://stackoverflow.com/questions/8023306/get-key-by-value-in-dictionary).

Can you abstract this into a `dict_key_for_value` function (with an appropriate docstring), so that it reads 
```python
question_name = dict_key_for_value(qa_dict, obj.filename)
```
i.e. function 1st arg is the dict, 2nd is the value and it returns the key.