Root no longer special, markup can contain multiple elements, recursively executing embedded scripts

The root task is no longer treated as special in the pipeline ac addon; instead it is serialized and flushed to the client, like all other tasks. The html frame now just prepends itself to the serialized tasks to be flushed.

A bug in the pipeline client was just appending the first element in a task's markup. Now all elements are appended.

Embedded scripts are not executed when they are inserted through innerHTML. Instead they need to be created using document.createElement. The fix involved recursively replacing the script elements with newly created script elements with the same content.