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 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 python code related to partitioning is in a new partitioning
  module;
* the neat & colorful partitions webview is a standalone HTML
  file in usr/share;
* plus other related minor simplifications & fixes.

Also closes #21.

Mostly the deleted lines from gtk_interface.py and installer.py have been migrated to partitioning.py with  refactoring for increased entropy and/or readability.