Fixed tests of region.py functions.
Added regions_to_mask() and NislImage class

Tests for regions_to_mask() have been written too.
Option background_label to regions_labels_to_list()

This option is used to give the label that should be considered as
background during the conversion (nothing is output for this label)
Added plot_regions_covariance.py (example file)

This script demonstrates usage of the regions API.