ENH: Add Time Frequency CSP

[kingjr]

It would be great to add in the decoding module, a time frequency class that exploits covariance of time frequency decomposed signals.

This implies

• deciding whether we go for a cospectral (i.e. fft decomposition) implementation versus a walelet decomposition
• optimizing for speed
• making an example.

[kingjr]

in mne-tools/mne-incubator#28