Drag&Drop with upload to server

Dropping images on a notebook uploads the image to a server and adds a
link in a new markdown cell
Update Python part of Drag-and-Drop extension

Added IPython startup script. This will start the websocket server
script with the port and notebook directory read from IPython
configuration
Update drag-and-drop

- make image upload work in Chrome
- dont't overwrite existing images
Update keycodes

Track changes from IPython: New location and names for keycodes.
Automatic startup

Allow automatic startup of the drag-and-drop extension by putting
`50-start-drag-and-drop.ipy`in the `startup` folder.
Merge pull request #45 from juhasch/dragdrop-upload

Dragdrop upload