Work around the regression introduced in matplotlib 1.3.1

Does what I proposed in #245. Emits the following warning (will be only printed once) when the user tries to plot a quantity array (or calls `ar.ravel()` directly):
> As a workaround for a bug in matplotlib 1.3.1, calling "ravel()" on a quantity will return unit-less values. If you get this warning during plotting, consider removing the units before plotting, e.g. by dividing by the unit. If you are explicitly calling "ravel()", consider using "flatten()" instead. The "ravel()" function will go back to returning a quantity after the matplotlib 1.4 release.