Cache loading of the HTML::Template layout.
  Propogate --force correctly.

  This was broken.
  Allow plugins to update the dependecies of files.

  This is now done for the file-inclusion plugin.
  We rebuild a page if any dependencies have changed.

  Now that the Templer::Site::Page object has a notion of dependencies
 we can use that list to see if a page needs to be rebuilt:

    * If the page changes.
    * If the selected layout changes.
    * If any included file changes.

  Closes #5.