This adds "dead" crumbs and rendering via a partial

Dead crumbs have no link and are only used for display purposes:

```
 crumb :users do
    link "Users", nil # 'dead' crumb, just used to indicate that we're in the users tree
    parent :root
  end

  crumb :user do |profile|
    link lambda {|profile| profile.user_name.truncate(20)}, user_path(profile), {title: profile.user_name}
    parent :users # etc...
  end
```

Also refactors the helper method to allow rendering via a partial.  Could possibly use a bit of polish, but for now it works.  This lets us separate the data (the tree of breadcrumb objects) from the display (the particular method of outputting content).  I tested to make sure the old method still works fine.

E.g. in application.html.haml (obviously, I use HAML but ERB works just fine too):

```
    #breadcrumbs
      = render_breadcrumbs "shared/breadcrumbs", :autoroot => true, :show_root_alone => true, :link_last => true
```

In app/views/shared/_breadcrumbs.html.haml:

```
%ul
  - crumbs.each do |crumb|
    %li= crumb_link(crumb)
```

Result:

```
<div id="breadcrumbs">
      <ul>
        <li><a href="/" title="Community Home">Home</a></li>
        <li>Users</li>
        <li><a href="/users/3" title="Banned User">Banned User</a></li>
      </ul>
    </div>
```