update compile script to work with new buildpack-env-dir API
update buildpack-env-dir tests
Cache everything one level deeper (inside $cache_dir/node) to avoid clobbering cached artifacts from other buildpacks.
Store node-version textfile in new .heroku directory, in the slug _and_ the cache.
Merge pull request #94 from heroku/clever-ponies-um

update compile script to work with new buildpack-env-dir API
Merge pull request #95 from heroku/namespaced-cache

Namespaced caching and node runtime version tracking
write a default npmrc
add a status message about npmrc
Merge pull request #98 from heroku/npm-cert-bug

npm cert bug
Merge https://github.com/heroku/heroku-buildpack-nodejs into HEAD

Signed-off-by: Ruth Byers <rbyers@pivotallabs.com>