ATLAS style for matplotlib

Hello, I have introduced an ATLAS style-like for matplotlib using [rcParams](http://matplotlib.org/users/customizing.html). Unfortunately not all the parameters can be controlled.

For this purpose I've added rootpy/plotting/style/atlas/style_mpl.py: it defines a dictionary to be used as mpl.rcParams.

To set the style I've added function set_style_mpl in rootpy/plotting/style/__init__.py. It takes as input a string or a dictionary.

I've provided a new example examples/plotting/plot_matplotlib_graph.py and I've changed examples/plotting/plot_matplotlib_hist.py removing some of the commands to set manually the style, since it is configured using rcParams.

I would like to add CMS and other styles. I have a chat with a CMS guy, he told me they don't have an official style...