Create a Python package for thumbnailer (setup.py)

[benoitbryon]

Convert current repository to a valid Python package, i.e. by adding a setup.py.

So that we can pip install thumbnailer, setup.py develop and take advantage of Python packaging.

[benoitbryon]

@Natim, can you review this 6-one-python-package branch?

I didn't make a pull request because I personally prefer the separated packages (#7 and #14) way. However, here is a branch you can review.

In this branch:

• created thumbnailer.core and thumbnailer.engines namespaces
• thumbnailer original package (the one developed in this repository) provides thumbnailer.engines.images and thumbnailer.engines.documents
• added a demo folder, which provides a custom thumbnailer.engines.same engine. As told by its name, this engine plugs in thumbnailer.engines namespace.

Installation requires some hack because #12 has not been implemented yet. But, it seems to work. Ooops, "worked once for me" in fact... tests are missing!

Still, I personally dislike the presence of PIL and docutils as thumbnailer dependencies (see thumbnailer's setup.py): if we just want to install custom engines like "thumbnailer.engines.same", we don't want dependencies of thumbnailer.engines.images and thumbnailer.engines.documents (i.e. PIL, docutils and their respective dependencies).