Plotting fixes

To make our plots a bit nicer:
- fixed the bug with axis labels alignment in tight_layout
- changed the font in plots from sans-serif to serif to make it consistent with Latex symbols (switch in CMS.py)
- added examples for plotting.py and CMS.py modules usage
- category suffixes in control plots (e.g. for PU systematics)
- rootpy_graph_bug_example.py updated from [this](https://github.com/rootpy/rootpy/issues/451) rootpy bugfix