Encryption improvements.

- Properly walk up the DOM to find:
  * contentEditable elements,
  * textarea's,
  * iframes with designMode == "on".
- This requires the latest git version of the SDK to work.
- We're being hit by bug 826080
Encryption mostly works.

- I still need to iron out the code that detects the element to be encrypted.
Improve behavior on iframe's with designMode=on.
User ownerDocument to make things more readable.
Get the body's contents, not the html's contents.

- Fixes a bug where parts of the style rules would appear along with the
originally encrypted message.
- Accents are still buggy, this is strange...