Remove unimplemented Url subclasses

Once upon a time these were supposed to help structuring your urls for
your page objects, but I'm not sure there's much to do that isn't
specific your needs. I can suggest a general pattern however:

// Make an abstract class for each host that you use, determine its
// host via an environment variable (so you can change this between
// test environments).
abstract class HostForApp1Url<T extends View> implements Url<T> {
  public String url() {
    return System.getProperty("host.for.app1");
  }
}

// For each application, extend the host that it lives on, and append
// the application's root path to that url, followed by whatever the
// specific page's path is that lives in that application.
class App1Url extends HostForApp1Url {
  String pagePath;
  public App1Url(String pagePath) {
    this.pagePath = pagePath;
  }
  public String url() {
    return super.url() + "/app1" + pagePath;
  }
}

// In the page object...
class MyPageInApp1 extends AbstractView {
  public static Url<MyPageInApp1> url() {
    return new App1Url("mypage.html", new MyPageInApp1());
  }

  // snip
}
Add UrlStringBuilder class to help construct proper urls
Update pom for sonatype requirements