Convert to proper Python module with setup.py, package, and tidy up the packaging

Hi

This pull request converts the shiftplanning api script to a properly packaged Python module in a fully backward compatible way.

It:

* renames `src` to `shift_planning`
* renames `shift_planning.py` to `shift_planning/__init__.py`, the module init file that populates the `shift_planning` namespace
* adds a `setup.py` to do an install with distutils
* Removes `simplejson` since that should not be under revision control in a 3rd party project. Adds instructions on how to install or download it, and adds a distutils dependency for it.
* adds a `.gitignore` to prevent versioning of `.pyc` files, build dirs
* Removes generated `.pyc` files from revision control

These changes tidy things up but have no direct effect on applications using the module. They'll just keep me sane when using it.