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Added a config file. Removed the old serv.list file. Added dinstall directory command line arguments.

This required changing the PkgInfo type from an alias for GenericPackageDescription 
to a record with a Repo and a GenericPackageDescription.
ResolvedPackage and ResolvedDependency also had to be changed to
keep a Repo rather than a tarball URL.

Use the package-url field in the GenericPackageDescription to record the tarball url for each package.

This adds a dependency on zlib, to unzip the downloaded 00-index.tar.gz.
Once unzipped, a small pure haskell tar reader is used to read
the index contents.
Servers in the server list now have names which are used 
as local cache directory names.
The --with-server command line flag has been removed since it
did not allow specifying a server name.

This is very untested, but should make cabal-install able to resolve
dependencies again after the move to Cabal 1.2.0. The package descriptions
are stored in a very hacky format atm the moment.

Enabled at -v2

This should make it all "Just Work"tm. We no longer need a Makefile
to do the installation. The point is, don't bother trying to use
/etc/cabal-install/srve.list as the global list. That's hard to do
because it involves installing that file, and we only want to do that
for global installs, so it's even more complex. Instead we should just
use cabal's facility to install data files. That way it ends up in
/usr/local/share/cabal-install-x.y/
or somewhere similar for a user install. But either way, cabal handles it.
The per-user config sill overrides this one, so it's still possible to edit
the list even if /usr/local/share/ is read-only. Also, we always use
$HOME/.cabal-install/ for the per-user config and package cache.
The one downside is that if an admin wants to use /etc/ and /var rather
than /root/.cabal-install/ then this is a tad harder. But the point is
at the moment it'll at least work for most people without fiddling,
including windows users.

cabal-install now puts the package list in /var/lib/cabal-install and the tarballs in /var/cache/cabal-install by default. Added command-line options for changing those.