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
it's actually very good but shrunk is normally better (it's the same as LW with cross-validated alpha param). There might be a few occasions in which LW performs be better. As it's fast it would be cheap to include it. We excluded it for simplification. But it's good to have it, even if only for debugging shrunk.