Issue 322/sub statement definition

[CookieCookson]

Fixes https://github.com/xapijs/xapi/issues/322

• Added missing version property to Statement
• Corrected to Timestamp type for stored and timestamp properties on Statement
• Added missing SubStatement interface (See https://github.com/adlnet/xAPI-Spec/blob/master/xAPI-Data.md#substatements)
• Corrected StatementObject union type to use SubStatement instead of Statement