Added an rc_context decorator to preserve rcParams.

Hi Olga,

Thanks for all the work you've put in prettyplotlib! I've given it a quick try over the last week and I really liked the enhanced looks of my graphs.

I tried to fix Issue #15 (making importing prettyplotlib side effect-free) since I wanted to use it along regular pyplot -using code. This solution fixes it, and seems to passes tests. Hope it can be useful!