Hotfixes (minor fixes)

Collections of minor fixes resulting from the update of the README, including:

• the method product (also available as cartesian_product) of Fromable relations, to build a CartesianProduct relation with another Fromable (similar to the (.*_)?join(_to)? methods of Fromable);
• a improved tokenize_values method of Tokenizers for pretty formatting embedded values
• a __eq__ and __repr__ method for _MISSING_TYPE
• a bug fix in Atom