Add AppCacheResourceTransformer

This change adds a new ResourceTransformer that helps handling resources
within HTML5 AppCache manifests for HTML5 offline application.

This transformer:
* modifies links to match the public URL paths
* appends a comment in the manifest, containing a Hash (e.g. "# Hash:
9de0f09ed7caf84e885f1f0f11c7e326")

See http://www.whatwg.org/specs/web-apps/current-work/multipage/offline.html#offline
for more details on HTML5 offline apps and appcache manifests.

Here is a WebConfig example:

  @Override
  public void addResourceHandlers(ResourceHandlerRegistry registry) {

    AppCacheResourceTransformer appCacheTransformer =
          new AppCacheResourceTransformer();

      registry.addResourceHandler("/**")
          .addResourceLocations("classpath:static/")
          .setResourceResolvers(...)
          .setResourceTransformers(..., appCacheTransformer);
  }

Issue: SPR-11964