Send SIGTERM to all Xvfb processes.

There is a dangling process bug in the code as it stands. Try this:
  In [1]: from xvfbwrapper import Xvfb
  In [2]: vdisplay = Xvfb(width=1280, height=720)
  In [3]: vdisplay.start()
    => [In another shell, check all Xvfb, you will find there are two. ]
  In [4]: vdisplay.stop()
    => [Check Xvfb, you will find one left.]

In the code in this commit, there is a group for all processes started by 
subprocess.  This group is then send SIGTERM thus killing all the processes 
started.