refactor repository and unit tests

[cknoll]

To make this repository more contribution friendly it should imho be structured in a "standard way", i.e. the top level directory only containing setup.py but not the actual source code. This simplifies the installation in development mode (pip install -e .) and also enables an easier overview of the repo for potential users and contributors.

Also this pr moves the bash based unit tests to a python file using the unittest module from python standard library. This should make it easier to add new tests (which are not necessarily command executions) in the future.

Note that make test still works., see e.g. https://app.circleci.com/pipelines/github/cknoll/pdftitle?branch=develop

[metebalci]

Thanks for the ideas here. I am implementing some of these in the next release, 0.12, with more reorganization/refactoring, hence I will not merge this request. The tests I have are actually not unit but end user tests, so I am not sure to change them to unittest.