New plugin: Volume

Added a plugin to gather some simple volume metrics (lvm).

It checks for the presence of lvm2app.h and if found it will compile in the module.

It basically will read any volume groups on the system and report the total size and amount free in GB.  

Note:  You will manually have to add any volume goup information in types.db
  ex. nova-volume             size:GAUGE:0:U, free:GAUGE:0:U