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?).
There is a missing space at the end of each string

('asdf'
 'ASDF')
==
'asdfASDF'

Also aligning vertically the beginning of each strings would look nicer.