Documentation: done migration from SGML to restructuredtext

[Nicolas Sebrecht]

Hi all,

I've done the migration to the new documentation. This is good news to
contributors which aim to help maintaining it.

The migration don't provide readable diffs to send here, so I've merge
it directly.

It is currently available at the homepage
https://github.com/nicolas33/offlineimap but is broken in any way, for
now.  Still don't know why (maybe github cache got stuck by the move).

There are still ways for improvements. Come to my mind:

  * refactoring
  * make it fully up-to-date
  * move away the gh-pages (or build them from the master documentation)
  * check and update internal links (some could be broken)

Now that we have a lightweight markup language for our documentation, I
hope more people will help to improve it.

Thanks reviewing it! ,-)
Enjoy!

-- 
Nicolas Sebrecht