Add ability to retrieve the default value of a Property after creation

One more pull request... I'm working on some tooling around Archaius that requires discovering the default values of properties specified in the code. This change exposes the default value of a property through an accessor method. I'm currently using some nasty reflection code to access the property class implementations' internals, but this API change would make the implementation much cleaner.

This change also ensures that the default values of the collection properties are effectively immutable by making a defensive copy.