feat(compiler): Support $baseUrl in HTML attributes when loading a template.

Angular fetches template HTML files outside of the browser's normal parsing flow. As a result, URLs in template files are interpreted relative to the root application, when the components defined by the template files are inserted into the DOM. This change enables a template author to prefix URLs with the string $baseUrl, which will be replaced with the relative base path of the template file.

So for an example template loaded from /component/foo/template.html:

<img src="$baseUrl/logo.png" />

becomes:

<img src="/component/foo/logo.png" />

Addresses #2384.

modules/angular2/test/render/dom/compiler/view_loader_spec.ts

@@ -118,6 +118,16 @@ export function main() {
                                });
            xhr.flush();
          }));
+      it('should replace $baseUrl in attributes with the template base url',
+         inject([AsyncTestCompleter], (async) => {
+           xhr.expect('http://ng.io/path/foo.html', '<img src="$baseUrl/logo.png">');
+           var view = new ViewDefinition({templateAbsUrl: 'http://ng.io/path/foo.html'});
+           loader.load(view).then((el) => {
+             expect(DOM.getInnerHTML(el)).toEqual('<img src="http://ng.io/path/logo.png">');
+             async.done();
+           });
+           xhr.flush();
+         }));
     });
 
     describe('css', () => {