Date-related classes: DateToString, DayOfMonth, DayOfWeek, DayOfYear, Millisecond, Minute, Month, Second, Week

Added DateToString.

Made the other classes use the FixedArityExpressionT class template with the appropriate number of arguments.

All these date classes implement an extract function and have been updated to Mongo 2.6.5.

May still require more test code.  Need to merge with FixedArityExpressionT in order to use them.  Mongo doesn't implement any test cases for these classes.