Fix fill randomly not working in geom_bar + geom_histogram

[jdanbrown]

This code randomly produces one of two graphs:

ggplot(diamonds, aes(x='clarity', fill='cut')) + geom_bar()

sometimes good	sometimes bad

Problem: geom._get_plot_args renames keys in mpl_params according to _aes_renames, but not in a deterministic order. In particular, geom_bar and geom_histogram both include renames:

• color -> edgecolor
• fill -> color

which when done in that order are fine but in the other order causes color to get dropped.

Solution: use OrderedDict in geom_bar and geom_histogram to ensure that the color -> edgecolor rename happens before the fill -> color rename.

[DSLituiev]

ggplot/geoms/geom.py, line 73 .items() need to be replaced with six.iteritems for backward compatibility

[jdanbrown]

Good call, but there are two other occurrences of .items() in this file that this change doesn't touch, and this change only turns one existing .items() into another, so what do you think about calling six.iteritems out of scope for this PR to keep the diff minimal (e.g. easier rebasing)?

[DSLituiev]

let us see what yhat people say. Opened issue #578.