Adding xvfb and xvfbwrapper

[chrisgorgo]

Closes #359 by providing a workaround consisting of framebuffer X server that can be used to render visualizations offscreen on headless nodes. Example:

from dipy.viz import window
from xvfbwrapper import Xvfb

vdisplay = Xvfb()
vdisplay.start()

ren = window.Renderer()
window.record(ren, n_frames=1, out_path='test.png', size=(600, 600))

vdisplay.stop()

Ideally (from user convienience point of view) Xvfb should be started automatically via bash wrapper script set as an ENTRYPOINT, but that could lead to some unforseen side effects so I would suggest leaving this for another PR.

[rosbo]

Thanks for adding tests :)

[TamasBaloghDev]

May God bless you @chrisgorgo for sharing this solution for the bad X server connection 😄