ENH: topo plot with white background and no axis when when colorbar=True

closes https://github.com/mne-tools/mne-python/issues/1525

cc @LauriParkkonen 

an example plot:

![topo_plot_w](https://cloud.githubusercontent.com/assets/3817535/4034321/87c2531c-2c84-11e4-9432-f115002ab4db.png)