Renamed functions for regions signals extraction

New naming scheme is
signals_to_img_(maps|labels)
img_to_signals(maps|labels)

"img" is a 4D-MRI image
"signals" are values taken by voxels, along scans.
Created _smooth_array()

This function is used to smooth an MRI image. It takes a numpy.ndarray as
input, and the corresponding affine.

A test has been added.
Added tests for NiftiLabelsMasker

Moved generate_fake_fmri() to testing.py
Fixed some bugs in NiftiLabelsMasker.

100% coverage on NiftiLabelsMasker (but mostly smoke tests).
Created image.high_variance_confounds()

This function does the same thing as signals.high_variance_confounds(),
but takes niimgs as input (and an optional mask) instead of signals.