Add support for application version

Juju 2.0 provides support for display of the version of
an application deployed by a charm in juju status.

Insert the application_version_set function into the
existing assess_status function - this gets called after
all hook executions, and periodically after that, so any
changes in package versions due to normal system updates
will also be reflected in the status output.

This review also includes a resync of charm-helpers to
pickup hookenv support for this feature.

Change-Id: If1ec3dcc5025d1a1f7e64f21481412ad630050ea