Union0 and Stringify

• Add a new Union0<T> interface, which only contains one possible type.
  To understand usage cases, see Designing with Types.
• Add Union0.Factory along with GenericUnions.nulletFactory() to create Union0.
• Add toString() implementation to generic unions

Release 1.0.0

Initial release

• Union1 through Union9
• GenericUnions
• Tests and Tennis kata