webhooks interchangable with callbacks in json
use the term webhooks in most tests
rename testing CallbackServer as WebhookServer
change create_job and shellcommand task to use word webhook
rename celery http_callback.HTTPCallbackTask as webhook.WebhookTask
remove currently unsupported `cancelled` webhook from schema
rename tests for ended webhook
add success and failure webhooks
`ended` is no longer a valid status reported by webhooks
The begun callback now reports its status as `running`
Merge pull request #23 from mkiwala/add_success_failure_webhooks

Add success failure webhooks (#15)
`callbacks` no longer an acceptable alias for `webhooks`
Merge pull request #24 from mkiwala/remove_callbacks_support

`callbacks` no longer an acceptable alias for `webhooks`