design message button

This is part of #164

We will need a new interface element for some upcoming tickets in the context of the news service: A message button. This Pull request provides this new interface element as it is needed by each one of these otherwise independent tickets.

I tested this button with chromium and firefox on linux and firefox and safari on mac. Not perfect, but looks ok.

![message_button](https://f.cloud.github.com/assets/202576/1713762/f53651a4-6192-11e3-8fe9-73721edfc4e1.png)

I would like to use icon fonts in the future to make sure the unicode characters we use are actually there. But that is another issue.