Interval deserialization fix

[insightindustry]

#80: Revised how the default deserializer functions for datetime.timedelta and datetime.datetime objects functions. When deserializing either, the default deserializer now starts by attempting to coerce the value to a datetime.timedelta using the Validator Collection timedelta() validator function, which supports the expression of amounts of time as both integers (e.g. 23 seconds) and strings (e.g. 00:00:23). If the object cannot be converted to a timedelta (if it is a complete / proper datetime with both a time and date value), the default deserializer will then revert to returning a datetime.datetime object.