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?).
At some point we were wondering what method we should use so I implemented both. After that, I added the specific `NILEARN_SHARED_DATA` variable. In the end, we just have a list of path with decreasing priority, so I left the colon split in place. I can remove it though.