This does a number of things beyond upgrading the basebox distribution
to Debian Wheezy:

* Drop Rakefile integration since it relies on Veewee's integration in
  Vagrant, which is broken. Now a workaround is documented.

* Install the 3.x "from"-port (squeeze's l-b 2.x re-versioned to 3.x
  to replace wheezy's real 3.x) of `live-build` from deb.t.b.org.

* Bump disk capacity to 15 GB to take into account the larger amount
  of space needed when building Wheezy images (#7133). This should
  also future-proof this aspect a bit.

* The documented process for buliding new baseboxes, in particular the
  part that gives each one a new name, not only *works* (as opposed to
  the old docs, and the Rakefile integration) but should also solve
  #6736. The different name implies that Vagrant won't use the cached
  version. Of course, the commit that updates the basebox must be
  present in a branch for it to use the new basebox.

* Various small improvements.