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