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 think that's fine, just deprecate both parameters then. I don't mind not matching the C code in the future if the mne-python methods are in 99% of the cases better, which I assume these are...? We should just say very clearly in docstring and examples when we have diverged. @agramfort WDYT about this policy?