add tightvnc support

[j0hnsmith]

With tightvncserver installed on a Ubuntu server I can use a vnc client to connect to the xvfb display by doing

export DISPLAY=:1
vncserver :1
python my_webdriver_script.py

then connecting to port 5901

It would be great if the same functionality could be added to pyvirtualdisplay somehow.

[ponty]

I added Xvnc support, check the example:

http://ponty.github.com/PyVirtualDisplay/usage.html#vncserver

with Display(backend='xvnc', rfbport=5901) as disp:
    with EasyProcess('xmessage hello') as proc:
        proc.wait()