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
@agramfort you meant to post a line comment here saying this sounds good to you, right? :) I think it got posted to the `n_jobs = int | str` line debate above...