Start refactoring of the trigger system:

- Move each trigger handler into its own file
- Add dictionary helper classes for easier reading and writing of dict-based data
- Extract the web hook payload -> internal representation building for each trigger system
- Add tests for this transformation
- Remove support for Github archived-based building

buildtrigger/__init__.py

@@ -0,0 +1,5 @@
+import buildtrigger.bitbuckethandler
+import buildtrigger.customhandler
+import buildtrigger.githubhandler
+import buildtrigger.gitlabhandler
+