Add indication about markdown formatting in the publisher

This pull request add an indication at the bottom of the publisher to inform the user that he can use markdown to format his post. There is a link to https://diasporafoundation.org/formatting.

![markdown2](https://f.cloud.github.com/assets/930064/1535739/e7acc892-4ca7-11e3-9fee-11c1802dd40f.png)

The indication is hidden when the publisher is closed, so in this case, it looks exactly the same than before this PR.