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? 
@agramfort  Why FFT looks better not? Also 'iir' is not our default and I've had lot's of trouble with 'iir'. Using spectral plots before and after filtering I very rarely saw any edge artefacts / Gibbs window / ringig.