Default script/server to production mode

- `script/server` now sets RAILS_ENV=production if RAILS_ENV is not
  already set
- `script/bootstrap` precompiles assets when in production
- The production environment is set to serve static assets by default.
  This assumes that most folks won't be running DCIY behind any sort of
  reverse proxy.
- Add note about static content to README.

These changes address discussion in cobyism/dciy#22.