Merge lp://staging/~nick-moffitt/assets-bundler/use-jinja2-not-sed into lp://staging/assets-bundler

This change is untested as of now, but it relies on a jinja2 one-liner I've used in many places in the past. The basic trick is that it uses a template snarfed from stdin and populates it with environment variables. This approach isn't always the most robust, but it seems preferrable to a special-purpose script full of sed commands that aim to emulate jinja2's functionality.

The branch removes this script, which I hope will ease the maintenance burden. If the technique is not suitable here for some reason, I hope you will consider it in other situations where you need a quick template.