Add initial setup.py (#327)

Started to prepare an initial setup.py according to Python packaging best practices

• Supports package installation with a pmbootstrap entry point executable
• Supports pytest testing
• Moved all entry point logic into pmb module, had to fix some imports
• Dynamically resolves version from pmb/config/__init__.py using regex (see https://packaging.python.org/guides/single-sourcing-package-version/#single-sourcing-the-version)

Running pmbootstrap after installing it system-wide (or into a virtualenv) currently requires the usage of the -p aports flag. But this discussion is probably more relevant in #383 (closed).