new TFR code

starts to implement what is described in : https://github.com/mne-tools/mne-python/issues/1333

for now only sensor space analysis is investigated.

it also adds a new dataset (somato) well adapted to TF analysis.
one displays a topo like :

http://martinos.org/mne/dev/_images/plot_topo_channel_epochs_image_1.png

and topomap like:

http://martinos.org/mne/dev/_images/plot_evoked_topomap_1.png

I'll make the naming consistent though.