feat(core): separate refs from vars.

Introduces `ref-` to give a name to an element or a directive (also works for `<template>` elements), and `let-` to introduce an input variable for a `<template>` element.

BREAKING CHANGE:
- `#...` now always means `ref-`.
- `<template #abc>` now defines a reference to the TemplateRef, instead of an input variable used inside of the template.
- `#...` inside of a *ngIf, … directives is deprecated.
  Use `let …` instead.
- `var-...` is deprecated. Replace with `let-...` for `<template>` elements and `ref-` for non `<template>` elements.

Closes #7158

Closes #8264

modules/angular2/http.ts

@@ -56,7 +56,7 @@ export {URLSearchParams} from './src/http/url_search_params';
  *     <div>
  *       <h1>People</h1>
  *       <ul>
- *         <li *ngFor="#person of people">
+ *         <li *ngFor="let person of people">
  *           {{person.name}}
  *         </li>
  *       </ul>
@@ -194,7 +194,7 @@ export const HTTP_BINDINGS = HTTP_PROVIDERS;
  *     <div>
  *       <h1>People</h1>
  *       <ul>
- *         <li *ngFor="#person of people">
+ *         <li *ngFor="let person of people">
  *           {{person.name}}
  *         </li>
  *       </ul>