fixed a bug in statistics
BF: defult z-values are now reosnable. An alternative would be to use Nans.
ENH: better documentation of the contrast function
BF: convert frametimes to array upon design matrix creation
ENH: Change the default for GLM specification: `ols` instead of `ar1`
Merge pull request #303 from bthirion/glm_fixes

MRG: Glm fixes