Adding Flowdock as a supported publisher

This change adds support for publishing build status to Flowdock via the
Flowdock plugin (https://github.com/jenkinsci/flowdock-plugin). The
plugin gives the ability to publish directly to a flow via a (mandatory)
API token supplied by the user.

Change-Id: Ibceecfe19e05e40e13547257e45d9b826e197d4b

tests/publishers/fixtures/flowdock001.yaml

@@ -0,0 +1,3 @@
+publishers:
+  - flowdock:
+      token: abcdefghijklmnopqrstuvwxyzabcdef