Update scripts to be POSIX-compliant

This is a major change to the old bootstrap scripts. It adds POSIX-compliance and has better error catching functionality.
The scripts have also been split into smaller files for easier debugging, and to help the final bootstrap script avoid any complex / error-prone conditional logic.

Support Debian 7.5.0 and Ubuntu 14.04

This release adds official support for building Debian 7.5.0 and Ubuntu 14.04 appliances.

Removed support for Debian 6

Debian 6 will soon be unsupported and won't received security updates. We are removing it and encouraging everyone to use Debian 7.

Deprecated old CentOS 6.4 to fix UDEV issue

The previous release of CentOS 6.4 (i386 and x86_64) didn't have the udev fix, so an image would sometimes boot with a different MAC address assigned to eth0, thus creating an eth1 device and preventing SSH access.

This has been fixed and tested in release v0.1.7