NORB loader can now choose to load data/labels.
Tidied up in hdf5io.py after new API names.

Indentation was still still reflecting old names.
Removed duplicate import and tidied up.
Added ColorImageGrid for RGB images.

Also adjusted docs for ImageGrid.
Fixed critical bug in extract_patches.

It would ignore the parameter sample_per_image and extract all the
patches from the images. This causes much more correlated patches and
is thus a bug that could influence performance.