WIP: Regression examples / API

(continuation of https://github.com/mne-tools/mne-python/pull/1031, see also issue https://github.com/mne-tools/mne-python/issues/1016)

Examples of single-trial regression, written with a eye to an API function. This is a rebase of the old PR; the major additions are t value and p value maps in the sensor space regression. I'm also testing the results against statsmodels: all tests pass and the improvement in performance of statsmodels is pretty significant. The tests should probably not be in the examples eventually; most of the code in `lstsq` would hopefully get factored out in to an API function.