Mixer sensor

- Can read the volume of any OSS channel (use OSS mixer emulation if you're 
   using ALSA)
- Channel configurable by user
- Can set the volume by clicking or scrolling - for clicking to work, the
   width of the object that will be used to click is needed as the first
   argument; This must not be zero!

   If the sensor is called "mixer", then use 

   <label id="volume_control" x="52" y="13" width="125" height="10" 
      on-motion="mixer:move" on-click="mixer:click" on-scroll="mixer:scroll"/>

   provided there is a gauge watching mixer:volume at (52, 13), which is
   125 pixels wide. Also set 125 as the first argument to the sensor when
   it's instantiated.
- See comments in __init__.py for more
- For a display, look at volume.display in the psi-small displays package
