Adds support for 'update' to Entities.
Adds an example of checkbox declared as Entity with update method.
Merge branch 'patch-update-element' of github.com:LiveMinutes/firepad into LiveMinutes-patch-update-element
Remove entityHandle.update(), in favor of making .replace() more flexible.

.replace() will now automatically call an entity's update() method if it exists, else it'll fall back to just removing / recreating the entity as we did before.
Regenerate firepad.js.
Implement Firepad.Headless module
sendOperation hands errors to callback
Address feedback