"Add contact" button for unknown contacts

A button in the top of a conversation appears if the current contact is unknown to add this contact.
![screenshot_2014-06-03-23-46-17](https://cloud.githubusercontent.com/assets/5718007/3177432/1caa5bb0-ec10-11e3-8c2c-28052b695901.png)

rebase, this was already fixed recently