add CancelJob()

[tych0]

We'd like to be able to cancel jobs using this API, so add a wrapper for it.

[tych0]

=== RUN   TestStartStopTransientUnitAll
    methods_test.go:1284: Test unit found in list, should be stopped

seems unrelated to this patch? but i don't see how it could happen in any case... runStopUnit() waits on the job status channel...

[kolyshkin]

@tych0 if you're still interested, can you please rebase this? Also, you don't have to add Context suffix to the method name, it's only for those method that already have old (non-context) counterparts.