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?).
"3 simple ways" on l.18 should be replaced by "4 simple ways". You may need to expand the diff to see what I mean. No idea whether there is a better way to do it with github.