add initial support for dropping file through repo tabs

Drag your file into the target repository in the repo tabs
And it is done after a confirmation prompt for the user

![screen shot 2014-11-25 at 17 32 49](https://cloud.githubusercontent.com/assets/2123461/5180667/1f1d27e6-74c9-11e4-9333-fe5bbf306c94.png)