Rewrite

Rewrite of j-d-g

Goal: rewrite j-d-g's scripts (esp. build-and-provide-package and generate-git-snapshot) to avoid having one single script doing everything. Instead split into small modules which allow custom usage.

Identified issues:

• DIST vs distribution usage in *-source is confusing: https://github.com/mika/jenkins-debian-glue/issues/55
• Support sbuild (and possibly pbuilder, see https://github.com/mika/jenkins-debian-glue/issues/84) next to cowbuilder