ENH: Plot BEM contours on MRI

@agramfort: Is this close to what you were looking for? 
I suspect a bug with left vs right image flip

maybe it's because the affine of the image is not taken into account
look at:

nim.get_affine()

also I feel we should have a higher level function that only takes the subject name as parameter.

I'd love to be just able to type:

plot_bem(subject)

you could rename plot_bem_contours -> _plot_mri_contours making it private.

also I would make all lines yellow like freeview. It's the most visible color.