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? 
Because the data being filtered is short (epochs), would it not be better to use the FIR filter as default? The IIR adds ringing that might reduce the CFC a little bit, no?