Support AddMonths method in expressions

[mfenniak]

DateTime & DateTimeOffset's AddMonths method adds a specified number of months to a date time. If the date falls on an invalid day (eg. February 31st), it rolls the day number back to the last valid day for that month. Because of this logic and the fact that not all months are the same length, it is not trivial to implement AddMonths in our expressions.

It should be possible, though, to implement it as an expression that constructs a new time object with some logic. Maybe it's be easiest to implement as a JavaScript snippet that's automatically inserted into the expression tree by our expression builder... that'd be kinda cool.