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.
Default arg for kmeans
Changed k-means initialization: using normalized centroids.

Ultimately, the initial centroids should probably be an argument to the function; for now unit norm centroids is a general enough initial point?
Made ZCA epsilon a parameter.
Merge pull request #11 from clementfarabet/master

Made ZCA epsilon a parameter.
K-Means: resizes centroids depending on data shape.

Same idea as the whitening functions. This is quite convenient.
Also allowing the callback to break the learning loop.
BUGFIX: colouring functions do not modify input.