Add support to build snappy images in OBS. This means we need to parse snapcraft.yaml build description for dependencies, prepare data from remote resources and handle the build.

===Current State===

The good:

• OBS, osc and build script can build and publish .snap files

• A "snapcraft" source service (part of tar_scm) is able to parse snapcraft.yaml files and to prepare the needed external sources

• snap parts which are needed for building other snap builds can be prepared as standard binary package via new "part2pkg" source service, executed at build time.

Each snapcraft plugin (like qmake, cmake, maven) needs some special tooling. Half of them are supported now (esp. the important autotools, cmake and qmake ones since most GUI apps are using these and snap focuses on GUI apps in first place).

The example builds can be found here: https://build.opensuse.org/project/show/home:adrianSuSE:snappy

There is still some open work, but most of this is outside of this project:

• Getting a snap client working on openSUSE (WIP https://build.opensuse.org/project/show/home:adrianSuSE:branches:home:snapcraft)

• Add rpm support to snapcraft

• Support more snapcraft plugins. Not all of them will be possible, but that is okay, we promise reproducable builds in first place, not that every build descriptions out there will work out of the box.