Handlebars simplifications (second try)

Opening this PR in preference to #191.

---
This first change is to support a `data-requires="..."` attribute in templates to load specified AMD modules:

```xml
<template data-requires="delite/tests/unit/resources/TestWidget1, delite/tests/unit/resources/TestWidget2">
	<test-widget-1>hello</test-widget-1>
	<test-widget-2>world</test-widget-2>
</template>
```

This means that `delite/handlebars!` used as a plugin can compile any template, even if it contains widgets, without those widgets already being loaded.  It will help with builds and is probably a good idea in general.   The syntax is up for debate though.

Inadvertently fixes #159.

---

The second change, from @sbrunot's suggestion, is to create
`delite/Templated`, a mixin used by widgets with templates, in conjunction with `delite/handlebars!`.

Extending `delite/Templated` rather than `delite/Widget` has two effects:

(1) You specify the template via the `template` property rather than `buildRendering()` method:

```js
define(["delite/register", "delite/Templated", "delite/handlebars!./template/MyTemplate.html"],
   function (register, Templated, template){

	register("my-widget", [HTMLElement, Templated], {
		template: template
	});
});
```

instead of:

```js
define(["delite/register", "delite/Widget", "delite/handlebars!./template/MyTemplate.html"],
   function (register, Widget, template){

	register("my-widget", [HTMLElement, Widget], {
		buildRendering: template
	});
});
```

(2) It allows subclasses of a Templated widget to more easily inline a template definition:

```js
define(["delite/register", "delite/SuperClass"],
		function (register, SuperClass){

	register("my-sub-widget", [SuperClass], {
		template: "<template>hello world</template>"
	});
});
```

rather than:

```js
define(["delite/register", "delite/SuperClass", "delite/handlebars"],
		function (register, SuperClass, handlebars){

	register("my-sub-widget", [SuperClass], {
		template: handlebars.compile("<template>hello world</template>")
	});
});
```

However, this usage will likely require downloading handlebars to the client, even after a build, so it's not recommended except for RAD.