Add installation script

#!/bin/sh

set -e

if [ -z "${PREFIX}" ]; then

  PREFIX="/usr/local"

fi

BIN_PATH="${PREFIX}/bin"

SHARE_PATH="${PREFIX}/share/ruby-build"

mkdir -p "${BIN_PATH}"

mkdir -p "${SHARE_PATH}"

for file in bin/*; do

  cp "${file}" "${BIN_PATH}"

done

for file in share/ruby-build/*; do

  cp "${file}" "${SHARE_PATH}"

done

echo "Installed ruby-build at ${PREFIX}"