[Backend] Add server persistence

[mgreau]

For now, there is no persistence on the server side.
Each asciidoc and html5 source are sent to all connected users and each user can save the asciidoc source locally (thanks to indexedDB).

It could be interesting for a team to save their work on the server side.
There are differents approaches, one of them could be Elasticsearch as @lordofthejars explained on this blog post : http://www.lordofthejars.com/2013/06/searchable-documents-yes-you-can.html