Cache invalidation and handling of older Node versions

• Add clean_cache option for invalidating cache (respects cache contents of other buildpacks)
• Check if Node has been installed by a previous buildpack and raise an error informing user to remove that buildpack
• If Node version is changed, then we clean up older Node version and its node modules, bower components and npm

Initial tagged version

Tested with Phoenix 1.2 and should work with earlier versions of Phoenix.