added a basic partials support

One-line add that seems to work for my case - just added a second argument to the templates in the Handlebars.templates namespace, allows them to take an object of templates as partials, and then Handlebars works as usual

```
<!-- test.handlebars -->

<template name='test'>
	<ul>
            {{#each list}}
                <li>{{>list item}}</li>
             {{/each}}
        </ul>
</template>

<!-- listitem.handlebars -->

<template name='listitem' >
	<li>my name is {{this}}</li>
</template>

var listitem = Handlebars.templates['listitem'];
var html = Handlebars.templates['test']({ list : ['me', 'you', 'bobby sue']}, { listitem : listitem });
```

Still need to have each template in a separate file though, maybe I can dig in meteor code to see how to loosen that up