Move to package-style layout

The downloader script will be useful as a module too.
Move the two modules to a 'commonsdownloader' package.
Update unittests to match.
Fix import after module reorganisation

In the previous commit, we changed the module name
to thumbnaildownload. We had to change its import here.
Add download methods to commonsdownload

Some of the downloading logic was nested in the main method,
which is no good if we want to reuse them in a different context.

Move the logic to two methods, one for downloading from a text list,
one from a Python list of files.
Call these methods in the main.
Bump package version number
Merge branch 'master' of github.com:JeanFred/CommonsDownloader