Add support for private npm & bower repos

a la https://github.com/thirdiron/heroku-buildpack-nodejs. This allows private github repos to be used as dependencies for either npm or bower. Private npm dependency URLs must be in the form of `git+ssh://git@github.com:[user]/[repo].git`. Private bower dependency URLs must be in the form of `git@github.com:[user]/[repo].git`. Either npm or bower URLs may have a trailing `#semver`.
We need to retain scope through [line 175](https://github.com/jzempel/heroku-buildpack-ember-cli/blob/master/bin/compile#L175), right? So that `$GIT_SSH_KEY` is reference-able. At that point, most of your buildpack is scoped - which kind of defeats the point.