Mail overview: display the original message including a download link.

This PR fixes #439 

![bildschirmfoto 2014-08-05 um 10 39 02](https://cloud.githubusercontent.com/assets/485755/3808911/6369b986-1c7d-11e4-9fc8-cd7539f7d00d.png)

@lukasgraf please have a look.