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
You do not need to have a `verbose` decorator on this unless you plan on calling the private function at some point (or manually setting the `verbose` level when calling it), otherwise it just adds readability and execution overhead.