ENH: make plot_evokoed_topomap more flexible

addresses #1257 -- at least in part. We should tackle this at a deeper level during the sprint.

allows you to produce topmaps like these:

![screenshot 2014-05-03 22 53 40](https://cloud.githubusercontent.com/assets/1908618/2871346/b152f6ec-d305-11e3-9d1f-cd32af485487.png)

cc @fraimondo @agramfort @christianbrodbeck @Eric89GXL