More pythonic layout for the root package.

__init__ should only be a place for managing imports. The code related
to the parsing of the configuration file has moved to configuration.py.
Drop options for MongoClient.

There is no need to be that pedantic. Those options are for fine-tuning
only and can be misleading. Someone who wants to set those options would
clearly understand some advanced mechanism of MongoDB and can be
expected to know how **kwargs works in the skeleton. I don't think we
are making the life of this person harder by hiding those options. See
pep20.
Make the url of table_of_contents.xml tweakable

That url is the only link to the eurostat website that was hardcoded in
dlstats. Now that we can change that link in the configuration file, we
can serve a fake eurostat website to dlsats in order to ease the
development process. Specifically, the developer needs to be able to
create revisions to some time series on demand.