Add title to plot_evoked_topomap

rebased version of https://github.com/mne-tools/mne-python/pull/1032

is there really no way to specify the size of the frame using pixels instead of relative to its size? The title frame comes out huge when there's only one topomap (this isn't something that I did, it's always been this way, just that there was a huge empty space at the top).