setup_cabal.sh : provide some support for systems without apt-get

Instead of failing outright if the system does not have apt-get, we
perform checks to see if the desired programs are present on the system.
This also means that we relax the checking of installed programs to
using the `which` command instead of `dpkg`.

Signed-off-by: Pang Yan Han <pangyanhan@gmail.com>
setup_cabal.sh - clone Cabal to temporary folder instead of $HOME

Since the setup process is only done once, it is ok to clone the Cabal
to a temporary folder in case the user has a copy of Cabal at his/her
$HOME.

Signed-off-by: Pang Yan Han <pangyanhan@gmail.com>
README.md - update Setup instructions

Signed-off-by: Pang Yan Han <pangyanhan@gmail.com>