Add a module which creates the wsgi application

The app instance will be imported and served by uwsgi. The xsnippet
package itself won't be running a development server anymore - the
app will be created explicitly in manage.py later.
Make xsnippet.settings contain production defaults

xsnippet.settings should contain production default values, while
unit tests should have their own config.
Merge pull request #6 from xsnippet/wsgi_app

Wsgi app