This allows us to specify the exact version ranges of dependencies we
support, across all operating systems.

See issue #988.

This commit only adds the cabal file and declares the dependencies.
Enforcing them will be subject of a later commit.

So far, we have only given the names of dependency in most cases,
making sure that Ganeti builds with the dependency versions that are in
Debian.
This has caused problems for users that were not running Debian, or
when there were packages installed in the compiling user's ~/.ghc
directory, leading to compile-time type errors instead of pre-build
configuration errors; for an example, see:
  https://code.google.com/p/ganeti/issues/detail?id=979#c10

By explicitly listing our dependencies, we make clear which versions
are supported.

Modules and executables listed in the cabal file are automatically
generated from the Makefile; executables are symlinked in an apps/
directory to avoid repeated compilation as explained in:

  http://stackoverflow.com/a/6711739/263061
  http://stackoverflow.com/q/12305970/263061



For example, in apps/ there is:

  hluxid.hs -> ../src/hluxid.hs
  hluxid.hs.stamp
  [...]

Each executable symlink has a stamp file to track its modification time
because make follow symlinks and thus does not see symlink modification
times.

In addition to declaring our dependencies, this setup allows building
Ganeti's Haskell code with cabal to support editor integration and
cabal-based tooling, the Makefile remains the default way to build Ganeti.

Signed-off-by: Niklas Hambuechen <niklash@google.com>
Reviewed-by: Klaus Aehlig <aehlig@google.com>