add png visualization in ipython notebook

Add a _repr_png_ method to ghost to give a png preview of plots in the
notebook.
Update pygal/util.py
Merge pull request #16 from yobuntu/master

allow  unicode in labels
Merge pull request #15 from aebrahim/notebook

add png visualization in ipython notebook
x axis limiter for labels for line graph
Merge pull request #17 from wiktorn/unlimited_x

x axis limiter for labels for line graph