Add HBSPages filter

This filter supports building HBS pages to HTML. It also adds a couple new options:

* `layout` - an alias for `template`, to be a little clearer on what kind of template it is
* `layouts` - same, but for `templates`
* `defaultLayout` - a layout template to render into if no other template was specified

For example, my configuration now looks like:

```js
var templates = pickFiles('source/pages', {
  srcDir: '/',
  files: ['**/*.hbs'],
  destDir: '/'
});

templates = HBSPages(templates, {
  layouts: './source/layouts',
  helpers: './helpers',
  partials: './source/partials/',
  globals: {
    data: data
  },
  defaultLayout: 'default.hbs'
});
```

Also added an example Handlebars file and tweaked the head partial to use the `title` and `description` fields :)