Increase the helper VM's default RAM size to 512MB

The default Virtual RAM size for the helper VM used to be 256MB
Remove obsolete dependency to qemu-img

snf-image does not make any use of this utility any more
Remove chntpw and reglookup in favor of hivex*

Use hivexget and hivexregedit instead of reglookup and chntpw to browse
and alter the windows registry. The hivex utilities are better designed
and maintained
helper: add missing program checks in ./configure

Add checks for tune2fs, sfdisk, sgdisk and eatmydata
docs: Add a configuration section

Add a section where it is explained how snf-image can be configured
via the /etc/default/snf-image file.
docs: Update installation section

 * Add instructions for manually creating `/var/lib/snf-image/helper'
   directory when installing the software from source.

 * Remove instructions for installing Debian Squeeze packages (Debian
   Squeeze is no longer supported)
Do a spell-check on the project's files

 * Fix various spelling errors
 * Add module comments on all python utilities