Fixed order of notebook loading and kernel starting. For security reasons, the kernel should not be started until after the notebook content is completely loaded and on the page. This prevents people from creating notebooks that run nasty code on the users machine at load time. In order to implement this, we had to create a CodeCell.set_kernel method that allows the kernel attribute of a CodeCell to be set at a later time. This also fixes some error messages we were seeing related to the kernel's channels not being setup properly when a send was attempted.