559 make media api more consumer friendly

I have added 'field_xpath' field on the media object which was the remaining part of #559 
![screen shot 2014-11-06 at 3 02 18 pm](https://cloud.githubusercontent.com/assets/1128227/4935464/32caa0aa-65ad-11e4-9922-a43a32692759.png)

closes #559