add toString method for atom classes

lib/domain/atom_category.dart

@@ -13,4 +13,13 @@ class AtomCategory {
     var label = element.getAttribute("label");
     return new AtomCategory(term, scheme, label);
   }
+
+  @override
+  String toString() {
+    return '''
+      term: $term
+      scheme: $scheme
+      label: $label
+    ''';
+  }
 }

lib/domain/atom_feed.dart

@@ -78,4 +78,23 @@ class AtomFeed {
         rights: rights,
         subtitle: subtitle);
   }
+
+  @override
+  String toString() {
+    return '''
+      id: $id
+      title: $title
+      updated: $updated
+      items: $items
+      links: $links
+      authors: $authors
+      contributors: $contributors
+      categories: $categories
+      generator: $generator
+      icon: $icon
+      logo: $logo
+      rights: $rights
+      subtitle: $subtitle
+    ''';
+  }
 }

lib/domain/atom_generator.dart

@@ -13,4 +13,13 @@ class AtomGenerator {
     var value = element.text;
     return new AtomGenerator(uri, version, value);
   }
+
+  @override
+  String toString() {
+    return '''
+      uri: $uri
+      version: $version
+      value: $value
+    ''';
+  }
 }

lib/domain/atom_item.dart

@@ -73,4 +73,22 @@ class AtomItem {
         summary: summary,
         rights: rights);
   }
+
+  @override
+  String toString() {
+    return '''
+      id: $id
+      title: $title
+      updated: $updated
+      authors: $authors
+      links: $links
+      categories: $categories
+      contributors: $contributors
+      source: $source
+      published: $published
+      content: $content
+      summary: $summary
+      rights: $rights
+    ''';
+  }
 }

lib/domain/atom_link.dart

@@ -22,4 +22,16 @@ class AtomLink {
     }
     return new AtomLink(href, rel, type, hreflang, title, length);
   }
+
+  @override
+  String toString() {
+    return '''
+      href: $href
+      rel: $rel
+      type: $type
+      hreflang: $hreflang
+      title: $title
+      length: $length
+    ''';
+  }
 }

lib/domain/atom_person.dart

@@ -14,4 +14,13 @@ class AtomPerson {
     var email = xmlGetString(element, "email", strict: false);
     return new AtomPerson(name, uri, email);
   }
+
+  @override
+  String toString() {
+    return '''
+      name: $name
+      uri: $uri
+      email: $email
+    ''';
+  }
 }

lib/domain/atom_source.dart

@@ -15,4 +15,13 @@ class AtomSource {
 
     return new AtomSource(id, title, updated);
   }
+
+  @override
+  String toString() {
+    return '''
+      id: $id
+      title: $title
+      updated: $updated
+    ''';
+  }
 }