templating welcome baskets

This MR offers a way to create updated Welcome baskets in a somewhat convenient way.

In order to do that, I created a python module which implements main parts of the document model and structural elements. It is located in the devtools/pybasket directory. Lacking a better name I gave it a plain name.

Moreover, the welcome directory contains the script createWelcomeBaskets.py. This file is the template for the welcome baskets. Right now, about 40% of the template is done. Two baskets are yet to include. But most importantly,

This MR is dependent on !32 (merged). The weaver tool will finally be used to encode basket sources files created by this MR.

In order to test this submission, you need to install the python module in your environment, e.g. with the provided setup.py in the devtools/pybasket/ directory. Then run createWelcomeBaskets.py in the welcome directory. This creates a welcome_source directory which can be 'weaved' to a welcome.baskets via the basketweaver tool. You'd need to run basketweaver --weave welcome_source --name welcome to create a new archive.

There aren't any localized strings marked yet. I don't have a clear picture yet how this is integrated in the current workflow.