Returning used disk space

iStat for iOS accepts an attribute (`u`) for used disk space in megabytes.  This PR adds in some quick math to pass back that attribute.  Screenshot attached shows difference on client.
![2014-09-28 at 12 04 am](https://cloud.githubusercontent.com/assets/39943/4433276/23183c2a-46cd-11e4-9e3d-fe7c748d072b.png)