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?).
I fixed it but in any case this page is just a draft and it's a bit outdated. We should decide if we maintain it or if we delete it.