Make platform.sh sensitive to all failures

Without "set -e", if you run this script using older cabal, it might continue running after failing to create a sandbox, which would install hptool's dependencies into user's local cabal installation.
Merge pull request #127 from gleber/patch-1

Make platform.sh sensitive to all failures
relax installer check to 10.6, since everything works there
re-vamp the package layout for OS X

for generic, use Cabal default layout
added missing things from hptool.cabal
more robust platform.sh

handles host cabal versions that are pre-sandbox support
checks for installed hptool version vs. build hptool version
update build notes