Merge branch 'master' into develop

bump version to 0.13next
docs: Replace Fedora 19 image links with Fedora 20
docs: Add missing SWAP image property section
docs: Change CentOS version from 6.0 to 6.x
docs: Change CentOS version from 6.x to 6
Replace README with updated README.md
docs: Fix some image source file URIs

In the image directives always use relative paths for images that are
hosted inside the project's repository.
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
disklabel.py: Rename Disklabel cls to Disk

The Disk class hosts the partitioning info of a Disk, namely an MBR
instance as well as a Disklabel instance.

For disklabels define 2 different classes:

 * BSDDisklabel
 * OpenBSDDisklabel

The first represents the traditional *BSD disklabel and the latter the
altered version found in newer releases of OpenBSD.

For now, only OpenBSDDisklabel is implemented.
disklabel.py: Add main() function & fix input opts

Add a main() function that works as entry point and rephrase the help
text in some input options.
disklabel.py: Add BSDDisklabel format fields

Add the partition table fields and the format field for the BSDDisklabel
class. The body of the class is not implemented yet.
disklabel.py: Abstract the PartitionTable class

Create a PartitionTableBase class that will host the common code for the
PartitionTable classes of the two Disklabel classes.
disklabel.py: Abstract the Disklabel class

 * Create a DisklabelBase class that the two Disklabel classes will
   subclass
 * Move common code to the base class
 * Cleanup the derived classes
disklabel.py: Add code for printing BSDDisklabels

Add basic support for traditional BSD disklabels