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
This reply is too short for me to understand. Are you saying that it can be a string, or it cannot? If it can, what can it be? It's not implied by the docstring.