Feature/mongo 2.6.5 expressions DateToString, DayOfMonth, DayOfWeek, DayOfYear, Millisecond, Minute, Month, Second, Week, Year

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.

DateToString is not a part of this.  It has been removed, though it is in a previous commit on this branch.