Some small improvements

1. Add a "total savings" output to the Gzip plugin. It's fun to see how much you save - my site loses 1.7MB.
2. Show a warning when default helpers or Compass are not installed because you're missing the dependency. For default helpers especially, it's confusing for them to disappear.
3. Require 'padrino-helpers' from the sitemap meta page. Now that it's not pulled in by default, the meta pages were crashing without the require. Now at least it'll be clearer that you need it.

It does seem that making 'padrino-helpers' an optional dependency has made things pretty complicated, since a lot of stuff depends on the helpers existing.