Allow uploading files by drag and drop

I've implemented simple file uploading by dragging it into the editor, similar to GitHub issues:

![screen shot 2014-02-05 at 17 40 23](https://f.cloud.github.com/assets/918034/2089233/5a9db78a-8e84-11e3-95ae-4754e8ebcdc7.png)

The link tag (e.g. `[[uploads/123.png]]`) is then inserted at the current cursor position.

Please view this as a first proposal, I'm sure there's some issues with this implementation. Also one could think about including a line at the bottom of the editor, just like GitHub does:

![screen shot 2014-02-05 at 17 45 24](https://f.cloud.github.com/assets/918034/2089272/efd0bb5e-8e84-11e3-8420-0d97ff126b78.png)