Add data model and alembic migrations

Add Node and Pacakge data model classes
Use Alembic to migrate/upgrade the db structure
Add configuration classes
	Use sqlite for development purposes

Refactor project structure:
- Database code now in a seperate file
- Models live in ponyexpress.models
- manage.py in the root path for proper module imports
Add relations to the data model

Node has Packages, Packages are installed on Nodes. Add the
appropriate migrations for the data model.

Signed-off-by: Christoph Kassen <c.kassen@telekom.de>