Improve object referencing

As mentioned in #22 and later discussed in greater detail on Gitter, we would like the ability to have direct references to objects that make up a SecurityRule instead of just their names in string form.

@btorresgil and I agreed this functionality should not change the way the library interprets or stores the config tree. So objects will still be children of the same parents they have always been. We are simply talking about creating references to those objects in the SecurityRule at this time.

This will most likely entail a "link_references" like method that the developer will invoke after the SecurityRule and the objects have been refreshed or otherwise loaded.

My thinking is that we add additional attributes to the SecurityRule class to not interfere with the present functionality. So for example, source will still contain a list of string names, but source_obj will be added to store a list of objects.AddressObject and objects.AddressGroup objects. This way, the read/write functionality of the SecurityRule object is not changed. We could also make it so that when an object is added or removed from the source_obj list, the same thing happens to its corresponding string name in source string list.

Obviously the above would apply to all currently represented objects in the objects module that have anything to do with a SecurityRule.

Start with the SecurityRule class and the objects module, then review the context from issue #22 and the referenced Gitter discussion. Define the scope across the represented objects that participate in SecurityRule, including how a link_references-style operation should run after loading. Done means direct object references are available without changing config-tree interpretation or existing read/write behavior.