MRG: 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
I would suggest and like to address this together with the deprecation issue. In my mind it somehow is connected / belongs / glues together. If we really deprecate in a few release cycles we either have a strong dependency on sklearn or we port code from sklearn.