types: add empty constructors for gson

src/org/fox/ttrss/types/Article.java

@@ -27,6 +27,10 @@ public class Article implements Parcelable {
 		readFromParcel(in);
 	}
 	
+	public Article() {
+		
+	}
+	
 	public Article(int id) {
 		this.id = id;
 		this.title = "";

src/org/fox/ttrss/types/Attachment.java

@@ -19,6 +19,10 @@ public class Attachment implements Parcelable {
 		readFromParcel(in);
 	}
 	
+	public Attachment() {
+		
+	}
+	
 	@Override
 	public int describeContents() {
 		return 0;

src/org/fox/ttrss/types/Feed.java

@@ -24,6 +24,10 @@ public class Feed implements Comparable<Feed>, Parcelable {
 		readFromParcel(in);
 	}
 	
+	public Feed() {
+		
+	}
+	
 	@Override
 	public int compareTo(Feed feed) {
 		if (feed.unread != this.unread)

src/org/fox/ttrss/types/FeedCategory.java

@@ -20,6 +20,10 @@ public class FeedCategory implements Parcelable {
 		this.order_id = 0;
 	}
 
+	public FeedCategory() {
+		
+	}
+	
 	@Override
 	public int describeContents() {
 		return 0;

src/org/fox/ttrss/types/Label.java

@@ -6,4 +6,8 @@ public class Label {
 	public String fg_color;
 	public String bg_color;
 	public boolean checked;
+	
+	public Label() {
+		
+	}
 }