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.
Removed unnecessary copy in colouring functions.
Merge pull request #13 from andreirusu/master

BUGFIX: colouring functions do not modify input
-x
Optimizing whitening/colouring functions.

Using expand() to avoid ger() and matrix alloc.