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?).
Just wondering, was this intentional? It seems like this colon-separated list of paths feature was slightly orthogonal to adding the NILEARN_SHARED environment variable.