Adding git-annex with missing dependencies.

This is a rather minimal build of git-annex, equivalent to the minimal build
from http://git-annex.branchable.com/install/cabal/

  - no assistant
  - no webapp
  - no webdav
  - no pairing
  - no xmpp
  - no dns

Signed-off-by: Magnus Therning <magnus@therning.org>