add openfmri_preproc script
add fetch_openfmri function in datasets
grant executable rights to openfmri_preproc
display help when no cmd line argument is passed
move preproc_dataset function from example to openfmri.py in the main lib. openfmri_preproc.py keeps the command line interface
Merge pull request #67 from schwarty/openfmri_example

[MRG] Openfmri example