Make a proper Python package

[osma]

Currently Annif is just a bunch of ad-hoc Python scripts in the root directory of a Git repo.
They should be turned into a Python package, probably by placing them in a directory called annif, writing an __init__.py script, plus metadata (setup.py, setup.cfg etc.) so the whole package can be installed using standard Python tools.

[osma]

Some scripts are utility-like (especially related to corpus building) and should probably be separated from the main package.