Inject ENV variables and allow for cache rebuild via ENV setting

Killing two birds with one stone:
closes https://github.com/linemanjs/heroku-buildpack-lineman/issues/24
closes https://github.com/linemanjs/heroku-buildpack-lineman/issues/25

We inject variables as suggested in:

https://devcenter.heroku.com/articles/buildpack-api

and then look for the presence of a CLEAR_HEROKU_BUILD_CACHE variable
as a flag to remove all of the cached NPM packages prior to doing "npm
install".