Namespaced caching and node runtime version tracking

update compile script to work with new buildpack-env-dir API

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.

Make config vars available to npm install

Set default NODE_ENV to 'production'. Can't sit on the fence forever with this implementation.

Blow away the cached node_modules directory instead of the entire cache.

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Create Procfile from `npm start`

If no Procfile exists and there is a scripts.start in package.json, add
the scripts.start script as the 'web' process type to a new Procfile.

only prune when node_modules directory is restored from cache. Fixes #75

Add support for custom .npmrc