generate a warning message when backslashes in namespaces are not escaped properly

Running a build now procudes an output like this if backslashes are not escaped (see #11):

```text
sphinx-build -b html -d _build/doctrees   . _build/html
Running Sphinx v1.1.3
loading pickled environment... done
building [html]: targets for 1 source files that are out of date
updating environment: 0 added, 1 changed, 0 removed
reading sources... [100%] components/event_dispatcher/container_aware_dispatcher                                                                                        
/var/www/Symfony/docs/components/event_dispatcher/container_aware_dispatcher.rst:10: WARNING: backslash not escaped in :class:`Symfony\\Component\EventDispatcher\\ContainerAwareEventDispatcher`
/var/www/Symfony/docs/components/event_dispatcher/container_aware_dispatcher.rst:47: WARNING: backslash not escaped in :method:`Symfony\\Component\\EventDispatcher\ContainerAwareEventDispatcher::addListenerService`
looking for now-outdated files... none found
pickling environment... done
checking consistency... done
preparing documents... done
writing output... [100%] index                                                                                                                                          
writing additional files... genindex search
copying static files... done
dumping search index... done
dumping object inventory... done
build succeeded, 2 warnings.

Build finished. The HTML pages are in _build/html.
```