Add dateOperator extensions

when using grop/project phase I'd to have ability to use typed extension instead of passing bson document

var project =
BsonDocument.Parse(
"{Key:1, Timestamp:1, year:{$year:'$Timestamp'}, dayOfYear:{$dayOfYear:'$Timestamp'}}");
var group = BsonDocument.Parse("{_id:

{year:'$year', dayOfYear:'$dayOfYear'}

, count:{$sum:1}}");
var result = collection.Aggregate().Project(project).Group(group).ToList();

in this case i will expect that project will look like this:

.Project(i => new

{ i.Key, dayOfYear = i.Timestamp.GetDayOfYear, year = i.Timestamp.GetYear }

):