Fixed disqus_helper in mako templates

Page title is escaped safely with json.dumps(..) to support titles
containing quotes
Update nikola/plugins/command_init.py

Now, tuple!
Fixed disqus_helper in Jinja templates

To make it work, a new {{ ..|tojson }} filter is registered.
Merge pull request #234 from imankulov/disqus_fix

Fixed disqus_helper in mako templates