Implement GitHub issue to Pivotal Tracker story task hook

`AddPtTaskOnGhIssue` method of Workflow implements the specified hook. The
hook listens for `github.issues` events. When an event of this kind is
received, the issue body is check for Pivotal Tracker story URL. In case
a URL is found, a task is added to the relevant PT story containing the
link to the source GitHub issue.

Testing output:

```
$ ciderapp start -watch workflow
>>> Streaming logs for application workflow
[INFO] Logging service initialised
[INFO] PubSub service initialised
[INFO] Poblano API Basic authentication configured
[INFO] Hook enabled: GitHub issue created -> add Pivotal Tracker story task
[DEBUG] AddPtTaskFromGhIssue: Getting Poblano project record for repository workflow-testrepo...
[DEBUG] AddPtTaskFromGhIssue: Poblano project record received
[DEBUG] AddPtTaskFromGhIssue: Getting the Poblano user record for login tchap...
[DEBUG] AddPtTaskFromGhIssue: Poblano user record received
[INFO] AddPtTaskFromGhIssue: Pivotal Tracker story task created for GitHub issue https://github.com/tchap/workflow-testrepo/issues/33
```

Closes #1