Etherpad issues

- [x] Remove the `Share this pad` and `Save Revision` buttons from the Etherpad UI
- [x] Change the default pad text to be empty
- [x] Set the language of the pad to the current user's language
- [x] Add the headings plugin to the README file
- [x] Only generate an activity when the pad content has actually changed
- [ ] Preview generation for collaborative documents is no longer working (see screenshot)

![screen shot 2013-05-13 at 12 49 59](https://f.cloud.github.com/assets/109850/495414/424e9d32-bbc3-11e2-9d4a-d90d08f856dc.png)