Add assert_raises_regexp in nilearn._utils.testing

for compatibility with Python 2.6. Also:
* move assert_warns to nilearn._utils.testing
* amended tests that were meant to use assert_raises_regexp
Stay clear from sklearn.utils.testing.assert_raises_regexp

sklearn 0.15 assert_raises_regexp is using match rather than search
which is very misleading. Definition copied and pasted from sklearn master
where this bug has been fixed
Merge pull request #319 from lesteve/add-assert-raises-regexp

Add assert_raises_regexp in nilearn._utils.testing