title: System.Import in Typescript not overriding the default path
body: <p>I am a newbie to Typescript and Angular2.I have an <code>app</code> folder where I am creating ts files and I am trying to generate transpiled 'js' files to another folder built. 'Js' files are being generated successfully but when I try to <code>import</code> the files to html using <code>System.import('built')</code>, scripts are not being refernced and throwing <code>404</code>. Both 'app' and 'built' folders are added to the root.
  Here is my tsconfig.json : </p>
  
  <pre><code>{
    "compilerOptions": {
      "target": "es5",
      "module": "commonjs",
      "moduleResolution": "node",
      "sourceMap": true,
      "emitDecoratorMetadata": true,
      "experimentalDecorators": true,
      "removeComments": false,
      "noImplicitAny": false,
      "outDir": "built"
    }
  }
  </code></pre>
  
  <p>And here I am importing the scripts to index.html :</p>
  
  <pre><code>System.import('built').catch(function(err){ console.error(err); });
  </code></pre>
  
  <p>Any help.</p>
  
  <p>Edit : Here I have added the systemjs.config.js:</p>
  
  <pre><code>(function (global) {
    System.config({
      paths: {
        // paths serve as alias
        'npm:': 'node_modules/'
      },
      // map tells the System loader where to look for things
      map: {
        // our app is within the app folder
        app: 'app',
        // angular bundles
        '@angular/core': 'npm:@angular/core/bundles/core.umd.js',
        '@angular/common': 'npm:@angular/common/bundles/common.umd.js',
        '@angular/compiler': 'npm:@angular/compiler/bundles/compiler.umd.js',
        '@angular/platform-browser': 'npm:@angular/platform-browser/bundles/platform-browser.umd.js',
        '@angular/platform-browser-dynamic': 'npm:@angular/platform-browser-dynamic/bundles/platform-browser-dynamic.umd.js',
        '@angular/http': 'npm:@angular/http/bundles/http.umd.js',
        '@angular/router': 'npm:@angular/router/bundles/router.umd.js',
        '@angular/forms': 'npm:@angular/forms/bundles/forms.umd.js',
        // other libraries
        'rxjs':                       'npm:rxjs',
        'angular2-in-memory-web-api': 'npm:angular2-in-memory-web-api',
      },
      // packages tells the System loader how to load when no filename and/or no extension
      packages: {
        app: {
          main: './main.js',
          defaultExtension: 'js'
        },
        rxjs: {
          defaultExtension: 'js'
        },
        'angular2-in-memory-web-api': {
          main: './index.js',
          defaultExtension: 'js'
        }
      }
    });
  })(this);
  </code></pre>