WIP: Implementation of cross frequency coupling. 

Includes some functions to compute the cross frequency coupling and the modulation index as described in: - 

1. High gamma power is phase-locked to theta oscillations in human neocortex, Canolty 2006. (http://www.ncbi.nlm.nih.gov/pubmed/16973878)
 
2. Measuring phase-amplitude coupling between neuronal oscillations of different frequencies, Tort, 2010.
(http://www.ncbi.nlm.nih.gov/pubmed/20463205)

Things to be done: - 

1. Extensive testing of implementation with reliable sample data. 
2. Improve API, make more mne (structure, documentation, logging etc.)
3. Decide how to integrate / use with raw and epochs. 

Any suggestions/ideas on how best to do this? 
The one array would be traces_plot which is the ztraces corrected for the z_threshold value. 

z_threshold is computed from ztraces based on alpha (FDR error rate) value, if that is changed, then recomputation is needed. 

FDR correction can be done outside the cfc function, then we would have to pass ztraces and alpha (FDR error rate) as input for the plotting function. It would reduce 1 input and 1 output parameter for cfc function. But I would not go for it because test for significant cfc is not only dependent on the alpha/z_threshold value but also the frequency range used and recomputing it would be the best approach.