WIP Example implementing DICS pSPM calculation

This example is intended to form a basis from which to implement DICS in
MNE-Python.
STY Use frequency mask instead of list of indices
Add missing call to pl.show()
Add handling of SSPs and whitening
ENH CrossSpectralDensity class and compute_csd

* Class for storing cross-spectral density, analogous to Covariance
* compute_csd computes cross-spectral density from epochs
Add compute_csd to __init__
Use compute_csd in example

* Example now uses epochs instead of raw data
* No longer doing whitening so that noise normalization is done as it is
  usually done in DICS (dividing by projected noise)
FIX Several small fixes

* Docstring correction
* Counter instead of len()
* Use deepcopy
* PEP8 and pyflakes fixes
WIP Compare CSD implementations

Several different CSD implementations (most commented out):

* Using nitime's Welch method
* Using nitime's periodogram_csd method
* With code copied from nitime's periodogram_csd method
* Using the multitaper module's _mt_spectra and _csd_from_mt
* Using only _mt_spectra (not commented out, active)
Simple CSD implementation using _mt_spectra
ENH Add multitaper mode for CSD calculation

Based on implementation in mne/connectivity/spectral.py
FIX Correct reading number of time points

Now works also in the case of tmin and tmax equal to None
Checks for input parameters
Tests for compute_csd
Allow tmax or tmin to be None when the other boundary isn't None
Small fix to matplotlib imports
Refactor dics example into dics_epochs()
DICS example uses dics_epochs
Separate apply_dics from dics_epochs
Return estimated time courses instead of source power