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

spring-webmvc/src/test/resources/org/springframework/web/servlet/resource/test/appcache.manifest

@@ -0,0 +1,17 @@
+CACHE MANIFEST
+
+# this is a comment
+CACHE:
+bar.css
+foo.css
+//example.org/style.css
+
+NETWORK:
+*
+
+CACHE:
+js/bar.js
+http://example.org/image.png
+
+FALLBACK:
+/main /static.html

spring-webmvc/src/test/resources/org/springframework/web/servlet/resource/test/error.manifest

@@ -0,0 +1,4 @@
+THIS DOES NOT START WITH "CACHE MANIFEST"
+
+CACHE:
+bar.css