Fixes an issue where incorrect code was being generated for an attribute whose only known value is an empty array.

Fixes issue where mixed-type attributes, or attributes whose only known value is null, had a corresponding Swift property of type [Any?]. Now the property's type is Any?.

Includes:

• Improved date parsing, thanks to @ole
• More helper methods in CreatableFromJSON extension, thanks to Andrew Duncan
• Improved README

Initial release of the json2swift tool.