NF: added a module to handle mixed-effects
ENH: added the corresponding tests
ENH: Added an example illustrating the impact of mixed-effects model
BF: Changed the mixed-effects doctest for Travis
BF: Changed the mixed-effects doctest for Travis -- again
Merge pull request #234 from bthirion/mixed-effects

MRG: Mixed effects

This PR adds a Python EM code for mixed-effects-based statistics computation (typically one- and two-sample tests, but also more general designs).
It does not contain the framework for voxel-level inference and permutation testing. These should arrive in a future PR.