General structure of the dune-buildbot

The repository should adhere to the following guide lines:

• It should only contain a python package dune.buildbot.
• The folder dune/buildbot should contain subdirectories that match the subdirs at https://github.com/buildbot/buildbot/tree/master/master/buildbot
• It should export a create_master script that supersedes buildbot create-master and configures a master from ini or cfg input.

This task can be closed once all existing content is brought into shape.