Regression

this PR is in reference to https://github.com/mne-tools/mne-python/pull/1034. It is addressing the API and style comments mentioned in that pull request. It is ready for review. 
I am not super enthusiast about the simulation of data.

can't we use a design matrix with 0 and 1 from epochs.events and epochs.selection and
contrast 2 conditions with a T or F test?