Add xkcd flag to remove_chartjunk

matplotlib supports to generate xkcd like plots since version 1.3.1.
They also look better if some of the chart junk is removed but not when
the linewidth of the axes that are shown is reduced.
Refactor loop/if in remove_chartjunk to use sets

for el in set_1:
    if el not in set_2:

This is the same as looping over each element in the difference of set 1
and 2
Merge pull request #58 from kain88-de/xkcd

Add xkcd flag to remove_chartjunk