remove unecessary comments
	modified:   census/stats.py
correct error due to commit 78ae5cb86677a5c2e9544bab01c8c9c8893c5744
which changed the way columns are described
	modified:   census/transformer.py
renaming of two cols
	modified:   census/meta.py
- plot_class_stat is now plot_cat_stat
- missing legend value corrected
	modified:   census/stats.py
- change to ValueMapper methods names
- add class Preprocess to handle preprocess on data
	modified:   census/transformer.py
- change mapper.mapping value order in __build_mapping_from_list
- add dummies variables building associated to mapper in Preprocess class

	modified:   census/transformer.py