Allow another data path

Fix issue #160.

A second environment variable, GLOBAL_NILEARN_DATA (name to be discussed) adds a path to dataset search. This should be used by administrators to provide a shared data folder (maybe NILEARN_SHARED would be a better name?).
We definitely need that functionality. That question is how to solve our problem best.

@lesteve : the problem that we are trying to solve is the usecase we a sysadmin install the data, or part of the data. Therefore there needs to be several layers of directories: one for the sysadmin, which won't be writeable by the end user, and one for the end user.