juju: moved collector tests to collector_test.go.

related to #553 and #537.
juju: initial code for IsReachable.

related to #537.
juju: initial code for IsReachable.

related to #537.
juju: added test for unreachable on 502.

related to #553 and #537.
juju: implemented IsReachable.

related to #553 and #537.