Change Log v1.6.1-Bson.txt

BSON library changes from 1.6 to 1.6.1

BsonTypeMapper.cs
    Map subclasses of BsonDocument correctly
    Special case mapping of objects that can be simply cast to BsonValue

BsonValue.cs
    Fix implementation of IConvertible for ObjectId

ObjectId.cs
    Now implements IConvertible

BsonClassMapSerializer.cs
    Handle serializing/deserializing null values for properties/fields that are declared as an interface