Recursive yield helpers

Hi,
I made a small fix that allows the yield helper to be used in a sub-template.
That solves the dynamic layout issue for me for issue #41 

So what does it bring you:
you can have a basic layout that just calls a named yield
```html
<template name="layout">
  {{yield 'design'}}
</template>
```

and have a named yield that calls the unnamed yield
*currently only the core layout can have named yields*
```html
<template name="mainDesign">
  {{yield}}
</template>
```

means: you can set up several routes with different designs around it
```coffeescript
Router.map ->
  @route 'main',
    path: '/'
    renderTemplates:
      'mainDesign':
        to: 'design'
  @route 'about',
    renderTemplates:
      'contentDesign':
        to: 'design'
```

if you have any questions, just drop me a line.