Transformer use_node removed (now default behaviour)
Add property formatters and validators. Fix #79

Remove format_guess function (replace by formatters).
Simplification of PropertiesContainer code.
Rename register_properties to register_canonical_properties.
find_properties current parsed node added to parameters.
Add more options to parse_numeral function.
Property.name is now Property.keys and can be multiple in a same property (based on named parameters in regexp).
Remove weak property of guess (replaced by WeakValidator class).
Added 'Extra' and 'Bonus' canonical forms to 'other' property.