Prevent the strange situation where trying to create tells you the room already exists, and when you try to join, the error is that the room doesn't exist
Since Chrome 34, a new API for screen sharing is available (the previous one using the flag will probably be deprecated soon)
This commit adds:
- Source of a simple Chrome extension to use this new API (which is a perfect copy of the sample extension given by &yet here: https://github.com/HenrikJoreteg/getScreenMedia
- Adapt help page
- New modal dialog to prompt user to install the extension from Google Web Store
- Better error messages if you can't share your screen
Should fix#7