Backport Chrome 34 workarround
Known bug in Chrome 34 for Linux
Update screen sharing

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