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()