Changed affine comparison between mask and image from exact equality to numpy.allclose in order to be stable to insignificant numerical perturbations
Added an intelligent logging function

logger.log walk up the call stack to find a BaseEstimator object and
display the name of the calling method. A verbosity level is also
supported.

Previous code has not yet been ported to this new system.
Updated NiftiMapsMasker to the new logging system

Run plot_adhd_covariance.py to see the behaviour.
Code cleanup
Code cleanup, updated tests of logger.log()
Merge branch 'master' of https://github.com/AlexandreAbraham/nilearn into kamitani_pr
Merge branch 'kamitani_pr' of https://github.com/AlexandreAbraham/nilearn into kamitani_pr