Use Cache directory & A fix to add location of R to PATH

I highly suggest adding my most recent commit today to update the PATH in bin/compile to include the location of R.  Without this other buildpacks cannot install anything which might depend on R -- specifically I was attempting to install rpy2 with the python buildpack (which failed because it couldn't find R).

The caching stuff (most of the changes) has been working for me, perhaps you'll like this as well.