Added labels in output of signals_to_img_maps()

When the mask_img parameter is not None, some region may be entirely
outside the mask and disappear. This output value tells the calling code
which regions have been kept and which have been dropped.

The behaviour is now consistent with that of signals_to_img_labels().
Added NiftiMapsMasker

Some basic tests have been created for NiftiMapsMasker.
A consistency test between region.img_to_signals_maps and img_to_signals_labels
  has been added.
Small fix.