WIP: use joblib memmapping pool

Added support for joblib memmapping pool, see joblib/joblib#44.  This will solve https://github.com/mne-tools/mne-python/issues/95

By default, memmapping is not used. I think it better to only enable it for functions where we know it makes sense. I added the use for memmapping to permutation_cluster_test() and changed the way the permutations are split (before  "_one_permutation" would only do a single permutation, which is very inefficient since joblib has to pickle/memmap the data n_permutations times).

I still need to add tests and add the support for this to  "permutation_cluster_1samp_test()" (and maybe other places where it makes sense).

PS: this should not be merged until  joblib/joblib#44 has been merged into joblib