- mapping is reversed : keys are string values, values are corresponding scalar
- mapper renamed mapping
	modified:   census/transformer.py
non_valids is now an attribute of ValueMapper object
	modified:   census/transformer.py
added mapping result
	modified:   census/transformer.py
returned dataframe of mapping_result is now a deep copy of original df
	modified:   census/transformer.py
added ValueMapper.get_col_info method
modified:   census/transformer.py