Rename _utils.fast_maths to _utils.extmath
Move is_spd from _utils.testing to _utils.extmath
Add tests for is_spd.
* test_fast_maths.py has been renamed to test_extmath.py
* Only use relative tolerance in is_spd to check that the matrix is symmetric
Add >= for nibabel version

to be consistent with other package versions
Fix nilearn directory creation
Merge pull request #261 from lesteve/move-is-psd-out-of-utils-testing

[MRG] Move is_psd outside of _utils.testing