closes #58:  build should fail if path to variables wasn't correct. bug.
closes #64:  Exceptions thrown from Callables aren't failing the build. bug.
closes #63:  drop <url default="">...</url> in favor of <default>...</default> as the default rewrite. If no <default> element exists, then just use the page name as the default. deprecation.
closes #40:  Allow boolean to prevent external url validation. enhancement.
closes #51:  link rel="canonical" isn't added by rewrites elements. bug.