hookenv.unit_get should offer facility to detect changed values

In a similar way that the Config object allows to check for changes in values by serializing them first, the unit_get() function should ideally provide such facility as well.

The case for this is changing public IP addresses, as described in https://lists.ubuntu.com/archives/juju/2014-December/004652.html

If the public IP address changes, the config-changed hook will be fired, but re-setting the IP address if it hasn't changed can have unwanted side-effects, and in order to avoid them, it would be nice to be able to have code like:

if unit_get().changed("public_ip"):
    do_something() # Forwarding the public IP to a client relation would be an obvious example.