Added pre-build hook for collecting information about build system.

Will be included as /var/lib/digabi/builder. File contains hostname,
kernel version and live-build version of the build system.

Fixes #41.
Updated apt-cacher-ng config for dev.digabi.fi.
Fixed bash completion for digabi commands.
Added check for required parameter.
Updated package (resized bootsplash to 640x480).