Bumped debian/ directory

This commit includes:
* bump debhelper and standards version to current,
* bump Python dependency to 2.7 (it's the default Python version
  in Debian stable; only ever going up from here),
* machine-readable copyright (DEP5),
* replaced rules script with simpler *.install files,
* renamed etc/init.d/live-installer -> debian/live-installer.init,
* dropping postinst/postrm (all that was needed before is now
  handled with dh_* or dpkg triggers).
Allow installation to more than one drive

This commit somewhat overhauls the partition selection screen.
It introduces the following changes:

* parted is used for easy, optimal-aligned partitioning;
* the HDD selection screen is no more,
* instead, all attached drives (sans the rootfs one) are shown
  in the disks (partitions) treeview,
* and there is a button 'Expert mode' for the "custom mount onto
  /target instructions";
* all the stuff related to partitioning is in a new partitioning
  module;
* the neat & colorful partitions webview is a standalone HTML
  file in usr/share;
* other related minor simplifications/fixes.

Also closes #21.
i18n of program and slideshow

This commit includes:

* i18n of slideshow in po4a system (with Debian-recommended
  paths),
* a .gitignore (finally),
* somewhat simplified slideshow.py and related code,,
* somewhat improved and updated slideshow text, partially joined
  with ubiquity's mint-slideshow texts,
* only some four "fuzzy" translations per language (kind to
  your translators),
* complete 'sl' translation as the contributer speaks it perfectly,
* removed (any reason for) en_GB and en_AU -- even po4a manual
  author thinks only two languages require additional locale
  specified: pt and zh,
* Makefiles for building/updating translations (target 'update'
  for updating program labels; slideshow is auto-updated on each
  build, just keep them *.po files in sync).