[ANN] share_layouts extension 0.3

I have committed a few changes to the share_layouts extension which
bring it to version 0.3.

* ShareLayouts::RailsPage is now just RailsPage
* You can create RailsPage models (which show up as "Application")
  in the page tree, which has these advantages:
      o You can create a dummy representation of a custom controller
        in the tree
      o You can use the page in generation of navigation, etc.
      o Any controller that uses a Radiant layout will search for a
        RailsPage that matches the current request URL to use as
        context for the layout, i.e. you can generate content with
        tags, snippets, etc.

This is not really a change, but I realized while working on this
version that it is possible to output Radius from your ERb templates,
which will then be rendered within the layout and is nice if you need to
use snippets or other built-in features of Radiant. Here’s to tighter