* Rework output when caching directories

This splits the logic when the buildpack saves and restores the cache
into two separate branches:

- when the user hasn't specified specific directories to cache (most everyone)
- when the user has given a list of specific directories to cache (power users)

This gives us more control over the default messaging, which has
included messaging for bower_components forever, even though most users
will never use bower, nor should we encourage them to.

These changes silence and minimize the messaging for the default case