-Added volume:selected and volume:freespace

Added selected volume and volume free space data, in response to the request in issue #153.

volume:selected returns the name of the selected volume as a string. I added it in the comments, but if the user hasn't selected a new volume, the cpu.selectedVolume.name returns blank. I added a quick fix there because I didn't want to start messing around too deeply in the code. 

volume:freespace returns the freespace of the current volume as an int, no problems.