Add AMD output support

Have added AMD support and the relevant test and updated the docs.

```js
handlebars: {
  compile: {
    options: {
      amd: true
    },
    files: {
      "path/to/result.js": "path/to/sources/*.hbs"
    }
  }
}
```

This will wrap your output an amd define function:

```js
define(function() {

this["JST"] = this["JST"] || {};

this["JST"]["test/fixtures/amd.html"] = function (Handlebars,depth0,helpers,partials,data) {
  helpers = helpers || Handlebars.helpers;
  


  return "<section class=\"main-app\">\n    <h1>Some title</h1>\n    <p>I've been compiled with amd support</p>\n</section>";};

  return this["JST"];

});
```