Support Importing NSDate from Unix Timestamp

I have added support for importing from JSON where dates are represented as Unix timestamps.

If a value that maps to an NSDate attribute in Core Data appears in the JSON as a string, the importing behavior is unchanged from before.

If the value appears in the JSON as a number and nothing is specified in the date attribute's userInfo for the key `dateFormat`, I assume the number represents the number of seconds since 1970 (10 digits). For JSON that represents its dates as the number of milliseconds since 1970 (13 digits), add the value `unixTime13` for key `dateFormat` in the userInfo.

I have provided tests.