python 3 errors in imports and syntax

[mfouesneau]

I was able to correct a couple issues and make simple plots.

Patch below

diff --git a/python/vaex/image.py b/python/vaex/image.py
index 70fe8a0..78f63fb 100644
--- a/python/vaex/image.py
+++ b/python/vaex/image.py
@@ -1,12 +1,15 @@
 __author__ = 'maartenbreddels'
 import PIL.Image
 import PIL.ImageDraw
-import StringIO
 import collections
 import numpy as np
 import matplotlib.colors
 import warnings
 from base64 import b64encode
+try:
+    from StringIO import StringIO
+except:
+    from io import StringIO

 def rgba_2_pil(rgba):
        with warnings.catch_warnings():
@@ -15,7 +18,7 @@ def rgba_2_pil(rgba):
        return im

 def pil_2_data(im, format="png"):
-       f = StringIO.StringIO()
+       f = StringIO()
        im.save(f, format)
        return f.getvalue()

@@ -37,7 +40,7 @@ cairo_modes["saturate"] = (lambda aA, aB: np.minimum(1, aA + aB),
 cairo_modes["add"] = (lambda aA, aB: np.minimum(1, aA + aB),
                                                   lambda aA, xA, aB, xB, aR: (aA*xA + aB*xB)/aR)

-modes = pdf_modes.keys() + cairo_modes.keys()
+modes = list(pdf_modes.keys()) + list(cairo_modes.keys())
 def background(shape, color="white", alpha=1, bit8=True):
        rgba = np.zeros(shape + (4,))
        rgba[:] = np.array(matplotlib.colors.colorConverter.to_rgba(color))

[maartenbreddels]

thx for the patch, I'll make sure it's covered by the unittests and apply this.

[maartenbreddels]

I added unittests that covers this, let me know if it works.