Regression

Closes #1322.

@t3on @TalLinzen don't be shocked if you don't recognize the code anymor, I simplified + cleaned up. With @agramfort we decided not to expose simulations (which might be mistakenly copied by users). We use a boring but simple to understand trial count instead. Also I felt that 3 functions for one simple thing is overkill. We have one that does it all (Epochs, SourceEstimates).
design_matrix : ndarray, shape  (n_observations, n_regressors)

to follow numpy convention