Refactored zca_layer to handle 2D, 3D & 4D inputs.
Refactored zca_colour and zca_layer tests to include 2D, 3D & 4D inputs.
Refactored test for zca_whitening to try 2D, 3D & 4D inputs.
Refactored pca_layer to support 2D, 3D & 4D inputs.
Refactored all pca whitening tests to check 2D, 3D & 4D inputs.
Added ZCA and PCA inverse tranform layers.
Added tests for inv_pca_layer and inv_pca_layer.
Updated inv_zca_layer_test to make sure it is equivalent to colouring.
Added a few convenience test functions which display whitened and coloured cifar10 data.
Merge pull request #9 from andreirusu/master

Added inv_zca_layer and inv_pca_layer export functions. Added 2D, 3D & 4D support for all whitening functions.