Plot Topomaps

As promised in https://github.com/mne-tools/mne-python/pull/392 this is the topomap plotting part of https://github.com/mne-tools/mne-python/pull/404.

You'd use it like

```python
path = mne.datasets.sample.data_path()
evoked = mne.fiff.read_evoked(path + '/MEG/sample/sample_audvis-ave.fif', 'Left Auditory')
mne.viz.plot_evoked_topomap(evoked, 0.1, ch_type='mag')
```