Add label sklearn estimator
    	dependencies : sklearn, scipy, numpy, skimage
    	The documents with labels are used to train a collection of
    	supervised classifiers.
    	Each label got a classifier (OneVsAll strategy)
    	The prediction are made at index time : non labelled documents
    	are filled with an estimated predict_label_list
    	The whoosh search takes into account the predict_label_list
    	Older documents may not be correctly predicted according to the
    	new document (online learning), so it can be usefull to reindex all
    	TODO: we might want to save the thumbnail in a cache file for each doc