Provide ability to specify id_ as parameter to cybox.core.Object #271

It would be very helpful if it was possible to specify an externally-generated identifier as the value of the ID for an Object instance when creating an instance of cybox.core.Object similar to how properties can be assigned to the instance. In python-cybox 2.1.0, the assignment of the externally-generated ID value must be done as a separate step as shown below:

obj = Object()
obj.id_ = "mycompany:Object-f24a7878-609f-ab5f-3073-44240209dd1a"

The python source code for the 'Object init()' appears to have a TODO to implement this.