Include jetty GzipFilter configuration in samples

I'd actually like to keep this an anonymous function here.  The purpose being, it's much more common to do something like this:

```clojure
(def custom-gzip (jetty-util/filter-holder (GzipFilter.) {"mimeTypes" "text/html,text/plain,text/xml,application/xhtml+xml,text/css,text/javascript,application/javascript,application/transit+json"
"minGzipSize" "0"
"deflateCompressionLevel" "9"})) 
```

And then in the service map do something like:
```clojure
::bootstrap/container-options {:context-configurator #(jetty-util/add-servlet-filter % {:filter custom-gzip})}
```

If you want to use the longer form for the anon function, the arg should be names `servlet-ctx`, since we often use `ctx` to mean the Pedestal context.  Linking to the ServletContextHandler Javadoc or the [Jetty ServletFilters doc](http://www.eclipse.org/jetty/documentation/current/advanced-extras.html) might be a good idea.

It's also worth mentioning that there's a `add-servlet-filters` for when you have more than one.