Standard plotting

This PR supersedes #2787. According to the discussion there, I have just copied @ChrisBeaumont suggestion from his ipython notebook into this code. As it turn out, his suggestion includes all the suggested improvements already. For most of them that should be obvious from the code, except for the issue of a standard font. This code does not touch the fonts so that we should get the matplotlib default, which is Vera, the font that is shipped with matplotlib and which is thus very likely to be present for all users.

Note that I did not include a test in the PR, because test will either be trivial (set parameter, check that parameter is set) or would require a pixel-by-pixel comparison to be meaningful. The first option is not very helpful, the second is overkill. 

CC (all people who commented on the original issue): @bourque @astrofrog @taldcroft @embray @eteq @ChrisBeaumont @mdboom @perrygreenfield 
can you change this name to ``set_astropy_matplotlib_style``?  After all, different plotting methods do exist...