Updated POD/documentation.

  No functional changes made.
  Load the modules we use.

  Templer::Site uses Templer::Site::Asset + Templer::Site::Page, but
 didn't require them - this caused test-case failure.
  New test - ensure assets are copied.

  This creates a stub site, and ensures that the assets are copied
 as we expect.  Specifically it tests:

    * .htaccess - a dotfile - is copied.
    * index.skx - a page source - is not copied.