Configuration change notification

[elandau]

Governator currently supports 'pull' based reading of @configuration annotated fields. It would be nice to add support for Archaius property change notification as well. I see two implementation options.

1. Support @configuration setProperty methods. The configuration binding phase will internally bind a listener to the method.
2. In addition to primitive and Supplier methods add a new ListenableSupplier class with which the user can register a callback function.