#534 improved login background customisation

- now login background can be defined via property
- readme updates

ng2-components/ng2-alfresco-login/src/components/alfresco-login.component.ts

@@ -44,6 +44,9 @@ export class AlfrescoLoginComponent {
     @Input()
     logoImageUrl: string;
 
+    @Input()
+    backgroundImageUrl: string;
+
     @Input()
     providers: string [] ;