juju run: relation context

This adds the ability to specify a --relation context with juju run.

     juju run --unit=haproxy/0 --relation=cache “hooks/cache-relation-changed”

Review at http://reviews.vapour.ws/r/15/
I think we may need to bump the API version here, and add backwards-compatibility checks. The existing JujuRunEndpoint will just ignore the new arguments.