Job cancellation logic not wired up in webmachine

[markan]

The internal state machine and client allows jobs to be canceled, but it was never wired up in the webmachine interface so there is not REST based API to cancel a job.

Internally tracking this as https://chefio.atlassian.net/browse/PUSH-63

Adding webmachine hooks should be pretty straightforward:
PUT to /pushy/jobs/
{ "job_id": ID,
"job_status": "cancel"
}

calls pushy_job_state:stop_job(JobId)