WIP: add option for automated regularization to mne.cov.compute_covariance

Todo:

- [x] show basic sensor examples / update existing examples
- [x] consolidate API including default values
- [x] deal with projs
No that does not work here. Auto will makr it more complex. I see it as advanced examplr

> On 18 Dec 2014, at 18:37, Teon <notifications@github.com> wrote:
> 
> In examples/inverse/plot_covariance_whitening_dspm.py:
> 
> > @@ -0,0 +1,146 @@
> > +"""
> > +===================================================
> > +Demonstrate impact of whitening on source estimates
> > +===================================================
> > +
> > +This example computes dSPM source estimates for the SPM faces data and compares
> > +proper regularization with insufficient regularization based on the methods
> > +described in [1]. The example demonstrates that insufficient regularization can
> > +lead to overestimation of source amplitudes.
> > +
> > +References
> > +----------
> > +[1] Engemann D. and Gramfort A. Automated model selection in covariance
> > +estimation and spatial whitening of MEG and EEG signals. (in press.)
> > +NeuroImage.
> i guess what i meant is that we could update that example with auto as an option if we want to highlight its usability in a simple case.
> 
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