Refactor Ledger Python module into Python package

[afh]

Allowing to extend the Python Ledger package with pure Python classes and methods as well as adding functionality to existing native classes using Python.

This is a draft PR as I'd like to get a conversation going about the pros and cons of the proposed change.

On the one hand this change will bring a lot of flexibility for future additions written entirely in Python and decouples the Python API for people using the Ledger Python package from what is exposed by Ledger's C++ native code via Boost.Python.

On the other hand this may break backwards compatibility, yet the change to migrate script is fairly straight-forward: Change import ledger to import ledger.core as ledger.

I'm not using ledger python much, so things may break there, for example it was necessary to change the module name in test/baseline/feat-value_py3.test from ledger to _core in order to make the tests pass.
There may be ways around it, I'm not sure yet.

[tbm]

Adding @davidsklar in case he has input.

[afh]

Closing this PR as as it has served its purpose to explore how to add a Ledger Python Package. I'm working on improving this further, but what work in progess is neither in draft nor review state at the moment.