ENH: Add option to show channel names in topomap

Add a parameter to plot_topomap and use it in plot_evoked_topomap and Evoked.plot_topomap.

Full channel names would probably be too much text, so I'm extracting only the channel numbers, but this might not be very robust.

What do people think?

![screen shot 2013-12-26 at 6 42 29 pm](https://f.cloud.github.com/assets/203573/1812661/16eb7e44-6e88-11e3-8442-aaa595df2257.png)