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
there seems to be heavy dependence on scikit-learn. Maybe you want to update the `getting started` page?

"Some isolated functions (e.g. filtering with firwin2) require SciPy >= 0.9 and scikit-learn >= whatever version".