application: introduce the Shaarli\Config namespace

application: introduce the Shaarli\Config namespace

Namespaces have been introduced with the REST API, and should be generalized
to the whole codebase to manage object scope and benefit from autoloading.

See:
- https://secure.php.net/manual/en/language.namespaces.php
- http://www.php-fig.org/psr/psr-4/

Signed-off-by: VirtualTam <virtualtam@flibidi.net>