Upgrade files feature

Hi,

I added some updates to the files section. It now allows uploads via drag&drop to the page. You can also select or drag&drop multiple files or even folders. It will also show a upload status bar for individual files. File delete from the index view is now done via Rails remote option to make it more quick.

![Screenshot](http://f.cl.ly/items/0343470w3j2T0x1A0H45/Bildschirmfoto%202014-09-16%20um%2021.57.35.png)

I also removed the little file widget that was in layouts, pages and snippets views in favor of loading the files section in a modal window. It will show a little icon (http://cl.ly/image/0m1d060L0L0o) in the main menu for that. That way you have all the features of the files page like drag&drop, edit, delete, categories and everything.

![Screenshot](http://f.cl.ly/items/1I3j2l000b2f440A2w3j/Bildschirmfoto%202014-09-16%20um%2021.51.45.png)

Maybe you like it.