refactor(build): simplify and modularize

simplify:
- use same html file for dart and JS
- build benchmarks automatically when doing `gulp build`
- centralize configuration

modularize:
- move all build tasks into separate node.js modules under
  `tools/build`.

changes:
- the `build` folder is now the `dist` folder

Closes #284

tools/build/runtime_paths.js

@@ -0,0 +1,14 @@
+System.paths = {
+  'core/*': '/core/lib/*.js',
+  'change_detection/*': '/change_detection/lib/*.js',
+  'facade/*': '/facade/lib/*.js',
+  'di/*': '/di/lib/*.js',
+  'rtts_assert/*': '/rtts_assert/lib/*.js',
+  'test_lib/*': '/test_lib/lib/*.js',
+  'reflection/*': '/reflection/lib/*.js',
+  'benchpress/*': '/benchpress/lib/*.js',
+  'examples/*': '/examples/web/*.js',
+  'benchmarks/*': '/benchmarks/web/*.js',
+  'benchmarks_external/*': '/benchmarks_external/web/*.js',
+};
+register(System);