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`.
I wasn't clear if scoping was right for this case since there is a lot more going on in your script between the ssh-agent launch and kill than there is in the script you referenced.