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? 
My bad, this supports only 1d data (1, n_times) at the moment, I have changed the docs to reflect the same. But we could think about how to make it accept raw data and how best the results can be visualized.