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? 
Checked it with fft, there is a slight difference in the modulation index values found.. Although with fft, I got a warning 
```
/home/sprav/.local/lib/python2.7/site-packages/mne-0.8.git-py2.7.egg/mne/filter.py:307: UserWarning: Attenuation at stop frequency 59.5Hz is only 4.7dB.
  '%0.1fdB.' % (att_freq, att_db))
```

With FFT filter
![phase_amplitude pa _plots_fft](https://cloud.githubusercontent.com/assets/4293133/5165587/e388d866-73e6-11e4-9903-4dbf22a427a1.png)


With IIR filter
![phase_amplitude pa _plots_iir](https://cloud.githubusercontent.com/assets/4293133/5165591/e71bf21a-73e6-11e4-8ebf-b1892a26063c.png)