SERVER-6239 support $add and $subtract with dates

Date - Date : NumberLong(Milliseconds between times)
Num  - Date : Error
Date - Num  : Date(original - Num as milliseconds)

Date + Date : Error
Num  + Date : Date(with num treated as millis)
Date + Num  : Same as Num+Date

Conflicts:

	jstests/aggregation/bugs/server6239.js
	jstests/aggregation/bugs/server6240.js
	src/mongo/db/pipeline/expression.cpp