New Liquid tag for listing files: directory

This is a follow up to @mojombo's response to @robru's [gallery pull request](https://github.com/mojombo/jekyll/pull/303).  I simplified the `name` and `dir` param to simply a `path` and changed the date-stamp files to use Jekyll's convention.  Usage is similar to @robru pull request:

```liquid
<ul>
  {% directory path: images/vacation %}
    <li> <img src="{{ file.url }}" /> </li>
  {% enddirectory %}
</ul>
```

It provides this data to the templates:

    file.url        # absolute path to file
    file.name       # basename
    file.date       # date extracted from beginning of filename, if present (optional)
    file.slug       # basename with the date and file extension stripped off
    file.title      # file.slug with hyphens converted to spaces, and put into Title Case

The `forloop` maintains [its usual context](http://wiki.shopify.com/UsingLiquid#For_loops).

My updates to this patch also removed the YAML globbing from files as that now throws errors for files without YAML.  Lastly I added an `exclude` param that rejects files based on a RegExp.  @mojombo wasn't a big fan of defaults, so the default exclusion of `/.html$/` might not fly...