Add a ResourceResolver implementation for WebJars

Prior to this commit, WebJars users needed to use versioned links within
templates for WebJars resources, such as `/jquery/1.2.0/jquery.js`.
This can be rather cumbersome when updating libraries - all references
in templates need to be updated.

One could use version-less links in templates, but needed to add a
specific MVC Handler that uses webjars.org's webjar-locator library.
While this approach makes maintaing templates easier, this makes HTTP
caching strategies less optimal.

This commit adds a new WebJarsResourceResolver that search for resources
located in WebJar locations. This ResourceResolver is automatically
registered if the "org.webjars:webjars-locator" dependency is present.

Registering WebJars resource handling can be done like this:

```java
@Override
protected void addResourceHandlers(ResourceHandlerRegistry registry) {
  registry.addResourceHandler("/webjars/**")
          .addResourceLocations("classpath:META-INF/resources/webjars")
          .resourceChain(true)
              .addResolver(new WebJarsResourceResolver());
}
```

Issue: SPR-12323

polish

spring-webmvc/src/test/resources/org/springframework/web/servlet/resource/META-INF/resources/webjars/foo/2.3/foo.txt

@@ -0,0 +1 @@
+Some text.