Install to /usr/local/bin.

Not every Mac user has a `~/bin` folder (added to their path). I have a `~/.bin` instead.

This PR allows installing from another path than the repository root, generalizes configuration variables, and installs self-destruct to `/usr/local/bin` -- without the `.sh` extension.

Also see: http://www.haskell.org/haskellwiki/Mac_OS_X_Common_Installation_Paths