Install udev from wheezy-backports.

Wheezy's udev is not compatible with Debian's 3.18 kernel, that removed
UEVENT_HELPER support.
Install consolekit and libck-connector0 from wheezy-backports.

udev from wheezy-backports breaks Wheezy's consolekit.
Force inclusion of the overlay kernel module in the initramfs.

This workarounds Debian#773868, for which I've just submitted a pull request.