Emacs integration update

Would you please consider pulling my changes back to your repo?

I made hsenv.el behave more like ``$HSENV/bin/activate'' script with regard to environment variables.

It will:
1) set PACKAGE_DB_FOR_* variables instead of PACKAGE_DB_FOR_GHC on activate;
2) unset variables on deactivate;
3) check GHC version to use appropriate option names.

Thank you!