rename plaintext cell -> raw cell
Raw cells should be *untransformed* when writing various output formats, as the point of them is to let users pass through IPython to their rendered document format (rst, latex, etc.). This is different from what is the logical meaning of 'plaintext', which would suggest that the contents should be preserved as unformatted plaintext (e.g. in a `<pre>` tag, or literal block).
In the UI, these cells will be displayed as 'Raw Text'.
WARNING: any existing v3 notebooks which use plaintext cells, when read in by versions after this merge, will silently rename those cells to 'raw'. But if such a notebook is uploaded into a pre-merge IPython, cells labeled as 'raw' will simply *not be displayed*.
Fernando Perez
0344f92a62