base: fix static files serving from DocumentRoot

* Implements Nginx's "try_files" like functionality using custom wsgi
  middleware for Apache and WSGIScriptAlias for invenio.wsgi together
  with mod_rewrite.

* NOTE regenerate your apache configuration by running
  `inveniomanage apache create-config`.

Signed-off-by: Jiri Kuncar <jiri.kuncar@cern.ch>

--
* closes #252
* see also http://blog.ionelmc.ro/2013/06/14/serving-files-if-they-exist-or-fallback-to-wsgi-app/